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28. Epidemiology and Statistics

28. Epidemiology and Statistics (12)

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28. Epidemiology and Statistics

Chapter Editors:  Franco Merletti, Colin L. Soskolne and Paolo Vineis


Table of Contents

Tables and Figures

Epidemiological Method Applied to Occupational Health and Safety
Franco Merletti, Colin L. Soskolne and Paolo Vineis

Exposure Assessment
M. Gerald Ott

Summary Worklife Exposure Measures
Colin L. Soskolne

Measuring Effects of Exposures
Shelia Hoar Zahm

     Case Study: Measures
     Franco Merletti, Colin L. Soskolne and Paola Vineis

Options in Study Design
Sven Hernberg

Validity Issues in Study Design
Annie J. Sasco

Impact of Random Measurement Error
Paolo Vineis and Colin L. Soskolne

Statistical Methods
Annibale Biggeri and Mario Braga

Causality Assessment and Ethics in Epidemiological Research
Paolo Vineis

Case Studies Illustrating Methodological Issues in the Surveillance of Occupational Diseases
Jung-Der Wang

Questionnaires in Epidemiological Research
Steven D. Stellman and Colin L. Soskolne

Asbestos Historical Perspective
Lawrence Garfinkel

Tables

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1. Five selected summary measures of worklife exposure

2. Measures of disease occurrence

3. Measures of association for a cohort study

4. Measures of association for case-control studies

5. General frequency table layout for cohort data

6. Sample layout of case-control data

7. Layout case-control data - one control per case

8. Hypothetical cohort of 1950 individuals to T2

9. Indices of central tendency & dispersion

10. A binomial experiment & probabilities

11. Possible outcomes of a binomial experiment

12. Binomial distribution, 15 successes/30 trials

13. Binomial distribution, p = 0.25; 30 trials

14. Type II error & power; x = 12, n = 30, a = 0.05

15. Type II error & power; x = 12, n = 40, a = 0.05

16. 632 workers exposed to asbestos 20 years or longer

17. O/E number of deaths among 632 asbestos workers

Figures

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29. Ergonomics

29. Ergonomics (27)

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29. Ergonomics

Chapter Editors:  Wolfgang Laurig and Joachim Vedder

 


 

Table of Contents 

Tables and Figures

Overview
Wolfgang Laurig and Joachim Vedder

Goals, Principles and Methods

The Nature and Aims of Ergonomics
William T. Singleton

Analysis of Activities, Tasks and Work Systems
Véronique De Keyser

Ergonomics and Standardization
Friedhelm Nachreiner

Checklists
Pranab Kumar Nag

Physical and Physiological Aspects

Anthropometry
Melchiorre Masali

Muscular Work
Juhani Smolander and Veikko Louhevaara

Postures at Work
Ilkka Kuorinka

Biomechanics
Frank Darby

General Fatigue
Étienne Grandjean

Fatigue and Recovery
Rolf Helbig and Walter Rohmert

Psychological Aspects

Mental Workload
Winfried Hacker

Vigilance
Herbert Heuer

Mental Fatigue
Peter Richter

Organizational Aspects of Work

Work Organization
Eberhard Ulich and Gudela Grote

Sleep Deprivation
Kazutaka Kogi

Work Systems Design

Workstations
Roland Kadefors

Tools
T.M. Fraser

Controls, Indicators and Panels
Karl H. E. Kroemer

Information Processing and Design
Andries F. Sanders

Designing for Everyone

Designing for Specific Groups
Joke H. Grady-van den Nieuwboer

     Case Study: The International Classification of Functional Limitation in People

Cultural Differences
Houshang Shahnavaz

Elderly Workers
Antoine Laville and Serge Volkoff

Workers with Special Needs
Joke H. Grady-van den Nieuwboer

Diversity and Importance of Ergonomics--Two Examples

System Design in Diamond Manufacturing
Issachar Gilad

Disregarding Ergonomic Design Principles: Chernobyl
Vladimir M. Munipov 

Tables

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1. Basic anthropometric core list

2. Fatigue & recovery dependent on activity levels

3. Rules of combination effects of two stress factors on strain

4. Differenting among several negative consequences of mental strain

5. Work-oriented principles for production structuring

6. Participation in organizational context

7. User participation in the technology process

8. Irregular working hours & sleep deprivation

9. Aspects of advance, anchor & retard sleeps

10. Control movements & expected effects

11. Control-effect relations of common hand controls

12. Rules for arrangement of controls

13. Guidelines for labels

Figures

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ERG040T1ERG040F1ERG040F2ERG040F3ERG040T2ERG040F5ERG070F1ERG070F2ERG070F3ERG060F2ERG060F1ERG060F3ERG080F1ERG080F4ERG090F1ERG090F2ERG090F3ERG090F4ERG225F1ERG225F2ERG150F1ERG150F2ERG150F4ERG150F5ERG150F6ERG120F1ERG130F1ERG290F1ERG160T1ERG160F1ERG185F1ERG185F2ERG185F3ERG185F4ERG190F1ERG190F2ERG190F3ERG210F1ERG210F2ERG210F3ERG210F4ERG210T4ERG210T5ERG210T6ERG220F1ERG240F1ERG240F2ERG240F3ERG240F4ERG260F1ERG300F1ERG255F1

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32. Record Systems and Surveillance

32. Record Systems and Surveillance (9)

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32. Record Systems and Surveillance

Chapter Editor:  Steven D. Stellman

 


 

Table of Contents 

Tables and Figures

Occupational Disease Surveillance and Reporting Systems
Steven B. Markowitz

Occupational Hazard Surveillance
David H. Wegman and Steven D. Stellman

Surveillance in Developing Countries
David Koh and Kee-Seng Chia

Development and Application of an Occupational Injury and Illness Classification System
Elyce Biddle

Risk Analysis of Nonfatal Workplace Injuries and Illnesses
John W. Ruser

Case Study: Worker Protection and Statistics on Accidents and Occupational Diseases - HVBG, Germany
Martin Butz and Burkhard Hoffmann

Case Study: Wismut - A Uranium Exposure Revisited
Heinz Otten and Horst Schulz

Measurement Strategies and Techniques for Occupational Exposure Assessment in Epidemiology
Frank Bochmann and Helmut Blome

Case Study: Occupational Health Surveys in China

Tables

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1. Angiosarcoma of the liver - world register

2. Occupational illness, US, 1986 versus 1992

3. US Deaths from pneumoconiosis & pleural mesothelioma

4. Sample list of notifiable occupational diseases

5. Illness & injury reporting code structure, US

6. Nonfatal occupational injuries & illnesses, US 1993

7. Risk of occupational injuries & illnesses

8. Relative risk for repetitive motion conditions

9. Workplace accidents, Germany, 1981-93

10. Grinders in metalworking accidents, Germany, 1984-93

11. Occupational disease, Germany, 1980-93

12. Infectious diseases, Germany, 1980-93

13. Radiation exposure in the Wismut mines

14. Occupational diseases in Wismut uranium mines 1952-90

Figures

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33. Toxicology

33. Toxicology (21)

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33. Toxicology

Chapter Editor: Ellen K. Silbergeld


Table of Contents

Tables and Figures

Introduction
Ellen K. Silbergeld, Chapter Editor

General Principles of Toxicology

Definitions and Concepts
Bo Holmberg, Johan Hogberg and Gunnar Johanson

Toxicokinetics
Dušan Djuríc

Target Organ And Critical Effects
Marek Jakubowski

Effects Of Age, Sex And Other Factors
Spomenka Telišman

Genetic Determinants Of Toxic Response
Daniel W. Nebert and Ross A. McKinnon

Mechanisms of Toxicity

Introduction And Concepts
Philip G. Watanabe

Cellular Injury And Cellular Death
Benjamin F. Trump and Irene K. Berezesky

Genetic Toxicology
R. Rita Misra and Michael P. Waalkes

Immunotoxicology
Joseph G. Vos and Henk van Loveren

Target Organ Toxicology
Ellen K. Silbergeld

Toxicology Test Methods

Biomarkers
Philippe Grandjean

Genetic Toxicity Assessment
David M. DeMarini and James Huff

In Vitro Toxicity Testing
Joanne Zurlo

Structure Activity Relationships
Ellen K. Silbergeld

Regulatory Toxicology

Toxicology In Health And Safety Regulation
Ellen K. Silbergeld

Principles Of Hazard Identification - The Japanese Approach
Masayuki Ikeda

The United States Approach to Risk Assessment Of Reproductive Toxicants and Neurotoxic Agents
Ellen K. Silbergeld

Approaches To Hazard Identification - IARC
Harri Vainio and Julian Wilbourn

Appendix - Overall Evaluations of Carcinogenicity to Humans: IARC Monographs Volumes 1-69 (836)

Carcinogen Risk Assessment: Other Approaches
Cees A. van der Heijden

Tables 

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  1. Examples of critical organs & critical effects
  2. Basic effects of possible multiple interactions of metals
  3. Haemoglobin adducts in workers exposed to aniline & acetanilide
  4. Hereditary, cancer-prone disorders & defects in DNA repair
  5. Examples of chemicals that exhibit genotoxicity in human cells
  6. Classification of tests for immune markers
  7. Examples of biomarkers of exposure
  8. Pros & cons of methods for identifying human cancer risks
  9. Comparison of in vitro systems for hepatotoxicity studies
  10. Comparison of SAR & test data: OECD/NTP analyses
  11. Regulation of chemical substances by laws, Japan
  12. Test items under the Chemical Substance Control Law, Japan
  13. Chemical substances & the Chemical Substances Control Law
  14. Selected major neurotoxicity incidents
  15. Examples of specialized tests to measure neurotoxicity
  16. Endpoints in reproductive toxicology
  17. Comparison of low-dose extrapolations procedures
  18. Frequently cited models in carcinogen risk characterization

Figures

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Sunday, 16 January 2011 19:15

Approaches to Hazard Identification: IARC

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The identification of carcinogenic risks to humans has been the objective of the IARC Monographs on the Evaluation of Carcinogenic Risks to Humans since 1971. To date, 69 volumes of monographs have been published or are in press, with evaluations of carcinogenicity of 836 agents or exposure circumstances (see Appendix).

These qualitative evaluations of carcinogenic risk to humans are equivalent to the hazard identification phase in the now generally accepted scheme of risk assessment, which involves identification of hazard, dose-response assessment (including extrapolation outside the limits of observations), exposure assessment and risk characterization.

The aim of the IARC Monographs programme has been to publish critical qualitative evaluations on the carcinogenicity to humans of agents (chemicals, groups of chemicals, complex mixtures, physical or biological factors) or exposure circumstances (occupational exposures, cultural habits) through international cooperation in the form of expert working groups. The working groups prepare monographs on a series of individual agents or exposures and each volume is published and widely distributed. Each monograph consists of a brief description of the physical and chemical properties of the agent; methods for its analysis; a description of how it is produced, how much is produced, and how it is used; data on occurrence and human exposure; summaries of case reports and epidemiological studies of cancer in humans; summaries of experimental carcinogenicity tests; a brief description of other relevant biological data, such as toxicity and genetic effects, that may indicate its possible mechanism of action; and an evaluation of its carcinogenicity. The first part of this general scheme is adjusted appropriately when dealing with agents other than chemicals or chemical mixtures.

The guiding principles for evaluating carcinogens have been drawn up by various ad-hoc groups of experts and are laid down in the Preamble to the Monographs (IARC 1994a).

Tools for Qualitative Carcinogenic Risk (Hazard) Identification

Associations are established by examining the available data from studies of exposed humans, the results of bioassays in experimental animals and studies of exposure, metabolism, toxicity and genetic effects in both humans and animals.

Studies of cancer in humans

Three types of epidemiological studies contribute to an assessment of carcinogenicity: cohort studies, case-control studies and correlation (or ecological) studies. Case reports of cancer may also be reviewed.

Cohort and case-control studies relate individual exposures under study to the occurrence of cancer in individuals and provide an estimate of relative risk (ratio of the incidence in those exposed to the incidence in those not exposed) as the main measure of association.

In correlation studies, the unit of investigation is usually whole populations (e.g., particular geographical areas) and cancer frequency is related to a summary measure of the exposure of the population to the agent. Because individual exposure is not documented, a causal relationship is less easy to infer from such studies than from cohort and case-control studies. Case reports generally arise from a suspicion, based on clinical experience, that the concurrence of two events—that is, a particular exposure and occurrence of a cancer—has happened rather more frequently than would be expected by chance. The uncertainties surrounding interpretation of case reports and correlation studies make them inadequate, except in rare cases, to form the sole basis for inferring a causal relationship.

In the interpretation of epidemiological studies, it is necessary to take into account the possible roles of bias and confounding. By bias is meant the operation of factors in study design or execution that lead erroneously to a stronger or weaker association than in fact exists between disease and an agent. By confounding is meant a situation in which the relationship with disease is made to appear stronger or weaker than it truly is as a result of an association between the apparent causal factor and another factor that is associated with either an increase or decrease in the incidence of the disease.

In the assessment of the epidemiological studies, a strong association (i.e., a large relative risk) is more likely to indicate causality than a weak association, although it is recognized that relative risks of small magnitude do not imply lack of causality and may be important if the disease is common. Associations that are replicated in several studies of the same design or using different epidemiological approaches or under different circumstances of exposure are more likely to represent a causal relationship than isolated observations from single studies. An increase in risk of cancer with increasing amounts of exposure is considered to be a strong indication of causality, although the absence of a graded response is not necessarily evidence against a causal relationship. Demonstration of a decline in risk after cessation of or reduction in exposure in individuals or in whole populations also supports a causal interpretation of the findings.

When several epidemiological studies show little or no indication of an association between an exposure and cancer, the judgement may be made that, in the aggregate, they show evidence suggesting lack of carcinogenicity. The possibility that bias, confounding or misclassification of exposure or outcome could explain the observed results must be considered and excluded with reasonable certainty. Evidence suggesting lack of carcinogenicity obtained from several epidemiological studies can apply only to those type(s) of cancer, dose levels and intervals between first exposure and observation of disease that were studied. For some human cancers, the period between first exposure and the development of clinical disease is seldom less than 20 years; latent periods substantially shorter than 30 years cannot provide evidence suggesting lack of carcinogenicity.

The evidence relevant to carcinogenicity from studies in humans is classified into one of the following categories:

Sufficient evidence of carcinogenicity. A causal relationship has been established between exposure to the agent, mixture or exposure circumstance and human cancer. That is, a positive relationship has been observed between the exposure and cancer in studies in which chance, bias and confounding could be ruled out with reasonable confidence.

Limited evidence of carcinogenicity. A positive association has been observed between exposure to the agent, mixture or exposure circumstance and cancer for which a causal interpretation is considered to be credible, but chance, bias or confounding cannot be ruled out with reasonable confidence.

Inadequate evidence of carcinogenicity. The available studies are of insufficient quality, consistency or statistical power to permit a conclusion regarding the presence or absence of a causal association, or no data on cancer in humans are available.

Evidence suggesting lack of carcinogenicity. There are several adequate studies covering the full range of levels of exposure that human beings are known to encounter, which are mutually consistent in not showing a positive association between exposure to the agent and the studied cancer at any observed level of exposure. A conclusion of “evidence suggesting lack of carcinogenicity” is inevitably limited to the cancer sites, conditions and levels of exposure and length of observation covered by the available studies.

The applicability of an evaluation of the carcinogenicity of a mixture, process, occupation or industry on the basis of evidence from epidemiological studies depends on time and place. The specific exposure, process or activity considered most likely to be responsible for any excess risk should be sought and the evaluation focused as narrowly as possible. The long latent period of human cancer complicates the interpretation of epidemiological studies. A further complication is the fact that humans are exposed simultaneously to a variety of chemicals, which can interact either to increase or decrease the risk for neoplasia.

Studies on carcinogenicity in experimental animals

Studies in which experimental animals (usually mice and rats) are exposed to potential carcinogens and examined for evidence of cancer were introduced about 50 years ago with the aim of introducing a scientific approach to the study of chemical carcinogenesis and to avoid some of the disadvantages of using only epidemiological data in humans. In the IARC Monographs all available, published studies of carcinogenicity in animals are summarized, and the degree of evidence of carcinogenicity is then classified into one of the following categories:

Sufficient evidence of carcinogenicity. A causal relationship has been established between the agent or mixture and an increased incidence of malignant neoplasms or of an appropriate combination of benign and malignant neoplasms in two or more species of animals or in two or more independent studies in one species carried out at different times or in different laboratories or under different protocols. Exceptionally, a single study in one species might be considered to provide sufficient evidence of carcinogenicity when malignant neoplasms occur to an unusual degree with regard to incidence, site, type of tumour or age at onset.

Limited evidence of carcinogenicity. The data suggest a carcinogenic effect but are limited for making a definitive evaluation because, for example, (a) the evidence of carcinogenicity is restricted to a single experiment; or (b) there are some unresolved questions regarding the adequacy of the design, conduct or interpretation of the study; or (c) the agent or mixture increases the incidence only of benign neoplasms or lesions of uncertain neoplastic potential, or of certain neoplasms which may occur spontaneously in high incidences in certain strains.

Inadequate evidence of carcinogenicity. The studies cannot be interpreted as showing either the presence or absence of a carcinogenic effect because of major qualitative or quantitative limitations, or no data on cancer in experimental animals are available.

Evidence suggesting lack of carcinogenicity. Adequate studies involving at least two species are available which show that, within the limits of the tests used, the agent or mixture is not carcinogenic. A conclusion of evidence suggesting lack of carcinogenicity is inevitably limited to the species, tumour sites and levels of exposure studied.

Other data relevant to an evaluationof carcinogenicity

Data on biological effects in humans that are of particular relevance include toxicological, kinetic and metabolic considerations and evidence of DNA binding, persistence of DNA lesions or genetic damage in exposed humans. Toxicological information, such as that on cytotoxicity and regeneration, receptor binding and hormonal and immunological effects, and data on kinetics and metabolism in experimental animals are summarized when considered relevant to the possible mechanism of the carcinogenic action of the agent. The results of tests for genetic and related effects are summarized for whole mammals including man, cultured mammalian cells and nonmammalian systems. Structure-activity relationships are mentioned when relevant.

For the agent, mixture or exposure circumstance being evaluated, the available data on end-points or other phenomena relevant to mechanisms of carcinogenesis from studies in humans, experimental animals and tissue and cell test systems are summarized within one or more of the following descriptive dimensions:

  •  evidence of genotoxicity (i.e., structural changes at the level of the gene): for example, structure-activity considerations, adduct formation, mutagenicity (effect on specific genes), chromosomal mutation or aneuploidy
  •  evidence of effects on the expression of relevant genes (i.e., functional changes at the intracellular level): for example, alterations to the structure or quantity of the product of a proto-oncogene or tumour suppressor gene, alterations to metabolic activation, inactivation or DNA repair
  •  evidence of relevant effects on cell behaviour (i.e., morphological or behavioural changes at the cellular or tissue level): for example, induction of mitogenesis, compensatory cell proliferation, preneoplasia and hyperplasia, survival of premalignant or malignant cells (immortalization, immunosuppression), effects on metastatic potential
  •  evidence from dose and time relationships of carcinogenic effects and interactions between agents: for example, early versus late stage, as inferred from epidemiological studies; initiation, promotion, progression or malignant conversion, as defined in animal carcinogenicity experiments; toxicokinetics.

 

These dimensions are not mutually exclusive, and an agent may fall within more than one. Thus, for example, the action of an agent on the expression of relevant genes could be summarized under both the first and second dimension, even if it were known with reasonable certainty that those effects resulted from genotoxicity.

Overall evaluations

Finally, the body of evidence is considered as a whole, in order to reach an overall evaluation of the carcinogenicity to humans of an agent, mixture or circumstance of exposure. An evaluation may be made for a group of chemicals when supporting data indicate that other, related compounds for which there is no direct evidence of capacity to induce cancer in humans or in animals may also be carcinogenic, a statement describing the rationale for this conclusion is added to the evaluation narrative.

The agent, mixture or exposure circumstance is described according to the wording of one of the following categories, and the designated group is given. The categorization of an agent, mixture or exposure circumstance is a matter of scientific judgement, reflecting the strength of the evidence derived from studies in humans and in experimental animals and from other relevant data.

Group 1

The agent (mixture) is carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.

This category is used when there is sufficient evidence of carcinogenicity in humans. Exceptionally, an agent (mixture) may be placed in this category when evidence in humans is less than sufficient but there is sufficient evidence of carcinogenicity in experimental animals and strong evidence in exposed humans that the agent (mixture) acts through a relevant mechanism of carcinogenicity.

Group 2

This category includes agents, mixtures and exposure circumstances for which, at one extreme, the degree of evidence of carcinogenicity in humans is almost sufficient, as well as those for which, at the other extreme, there are no human data but for which there is evidence of carcinogenicity in experimental animals. Agents, mixtures and exposure circumstances are assigned to either group 2A (probably carcinogenic to humans) or group 2B (possibly carcinogenic to humans) on the basis of epidemiological and experimental evidence of carcinogenicity and other relevant data.

Group 2A. The agent (mixture) is probably carcinogenic to humans. The exposure circumstance entails exposures that are probably carcinogenic to humans. This category is used when there is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals. In some cases, an agent (mixture) may be classified in this category when there is inadequate evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals and strong evidence that the carcinogenesis is mediated by a mechanism that also operates in humans. Exceptionally, an agent, mixture or exposure circumstance may be classified in this category solely on the basis of limited evidence of carcinogenicity in humans.

Group 2B. The agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans. This category is used for agents, mixtures and exposure circumstances for which there is limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals. It may also be used when there is inadequate evidence of carcinogenicity in humans but there is sufficient evidence of carcinogenicity in experimental animals. In some instances, an agent, mixture or exposure circumstance for which there is inadequate evidence of carcinogenicity in humans but limited evidence of carcinogenicity in experimental animals together with supporting evidence from other relevant data may be placed in this group.

Group 3

The agent (mixture or exposure circumstance) is not classifiable as to its carcinogenicity to humans. This category is used most commonly for agents, mixtures and exposure circumstances for which the evidence of carcinogenicity is inadequate in humans and inadequate or limited in experimental animals.

Exceptionally, agents (mixtures) for which the evidence of carcinogenicity is inadequate in humans but sufficient in experimental animals may be placed in this category when there is strong evidence that the mechanism of carcinogenicity in experimental animals does not operate in humans.

Group 4

The agent (mixture) is probably not carcinogenic to humans. This category is used for agents or mixtures for which there is evidence suggesting lack of carcinogenicity in humans and in experimental animals. In some instances, agents or mixtures for which there is inadequate evidence of carcinogenicity in humans but evidence suggesting lack of carcinogenicity experimental animals, consistently and strongly supported by a broad range of other relevant data, may be classified in this group.

Classification systems made by humans are not sufficiently perfect to encompass all the complex entities of biology. They are, however, useful as guiding principles and may be modified as new knowledge of carcinogenesis becomes more firmly established. In the categorization of an agent, mixture or exposure circumstance, it is essential to rely on scientific judgements formulated by the group of experts.

Results to Date

To date, 69 volumes of IARC Monographs have been published or are in press, in which evaluations of carcinogenicity to humans have been made for 836 agents or exposure circumstances. Seventy-four agents or exposures have been evaluated as carcinogenic to humans (Group 1), 56 as probably carcinogenic to humans (Group 2A), 225 as possibly carcinogenic to humans (Group 2B) and one as probably not carcinogenic to humans (Group 4). For 480 agents or exposures, the available epidemiological and experimental data did not allow an evaluation of their carcinogenicity to humans (Group 3).

Importance of Mechanistic Data

The revised Preamble, which first appeared in volume 54 of the IARC Monographs, allows for the possibility that an agent for which epidemiological evidence of cancer is less than sufficient can be placed in Group 1 when there is sufficient evidence of carcinogenicity in experimental animals and strong evidence in exposed humans that the agent acts through a relevant mechanism of carcinogenicity. Conversely, an agent for which there is inadequate evidence of carcinogenicity in humans together with sufficient evidence in experimental animals and strong evidence that the mechanism of carcinogenesis does not operate in humans may be placed in Group 3 instead of the normally assigned Group 2B—possibly carcinogenic to humans—category.

The use of such data on mechanisms has been discussed on three recent occasions:

While it is generally accepted that solar radiation is carcinogenic to humans (Group 1), epidemiological studies on cancer in humans for UVA and UVB radiation from sun lamps provide only limited evidence of carcinogenicity. Special tandem base substitutions (GCTTT) have been observed in p53 tumour suppression genes in squamous-cell tumours at sun-exposed sites in humans. Although UVR can induce similar transitions in some experimental systems and UVB, UVA and UVC are carcinogenic in experimental animals, the available mechanistic data were not considered strong enough to allow the working group to classify UVB, UVA and UVC higher than Group 2A (IARC 1992). In a study published after the meeting (Kress et al. 1992), CCTTT transitions in p53 have been demonstrated in UVB-induced skin tumours in mice, which might suggest that UVB should also be classified as carcinogenic to humans (Group 1).

The second case in which the possibility of placing an agent in Group 1 in the absence of sufficient epidemiological evidence was considered was 4,4´-methylene-bis(2-chloroaniline) (MOCA). MOCA is carcinogenic in dogs and rodents and is comprehensively genotoxic. It binds to DNA through reaction with N-hydroxy MOCA and the same adducts that are formed in target tissues for carcinogenicity in animals have been found in urothelial cells from a small number of exposed humans. After lengthy discussions on the possibility of an upgrading, the working group finally made an overall evaluation of Group 2A, probably carcinogenic to humans (IARC 1993).

During a recent evaluation of ethylene oxide (IARC 1994b), the available epidemiological studies provided limited evidence of carcinogenicity in humans, and studies in experimental animals provided sufficient evidence of carcinogenicity. Taking into account the other relevant data that (1) ethylene oxide induces a sensitive, persistent, dose-related increase in the frequency of chromosomal aberrations and sister chromatid exchanges in peripheral lymphocytes and micronuclei in bone-marrow cells from exposed workers; (2) it has been associated with malignancies of the lymphatic and haematopoietic system in both humans and experimental animals; (3) it induces a dose-related increase in the frequency of haemoglobin adducts in exposed humans and dose-related increases in the numbers of adducts in both DNA and haemoglobin in exposed rodents; (4) it induces gene mutations and heritable translocations in germ cells of exposed rodents; and (5) it is a powerful mutagen and clastogen at all phylogenetic levels; ethylene oxide was classified as carcinogenic to humans (Group 1).

In the case where the Preamble allows for the possibility that an agent for which there is sufficient evidence of carcinogenicity in animals can be placed in Group 3 (instead of Group 2B, in which it would normally be categorized) when there is strong evidence that the mechanism of carcinogenicity in animals does not operate in humans, this possibility has not yet been used by any working group. Such a possibility could have been envisaged in the case of d-limonene had there been sufficient evidence of its carcinogenicity in animals, since there are data suggesting that α2-microglobulin production in male rat kidney is linked to the renal tumours observed.

Among the many chemicals nominated as priorities by an ad-hoc working group in December 1993, some common postulated intrinsic mechanisms of action appeared or certain classes of agents based upon their biological properties were identified. The working group recommended that before evaluations are made on such agents as peroxisome proliferators, fibres, dusts and thyrostatic agents within the Monographs programme, special ad-hoc groups should be convened to discuss the latest state of the art on their particular mechanisms of action.

 

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Thursday, 10 March 2011 17:45

The Biological Basis for Exposure Assessment

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Workplace exposure assessment is concerned with identifying and evaluating agents with which a worker may come in contact, and exposure indices can be constructed to reflect the amount of an agent present in the general environment or in inhaled air, as well as to reflect the amount of agent that is actually inhaled, swallowed or otherwise absorbed (the intake). Other indices include the amount of agent that is resorbed (the uptake) and the exposure at the target organ. Dose is a pharmacological or toxicological term used to indicate the amount of a substance administered to a subject. Dose rate is the amount administered per unit of time. The dose of a workplace exposure is difficult to determine in a practical situation, since physical and biological processes, like inhalation, uptake and distribution of an agent in the human body cause exposure and dose to have complex, non-linear relationships. The uncertainty about the actual level of exposure to agents also makes it difficult to quantify relationships between exposure and health effects.

For many occupational exposures there exists a time window during which the exposure or dose is most relevant to the development of a particular health-related problem or symptom. Hence, the biologically relevant exposure, or dose, would be that exposure which occurs during the relevant time window. Some exposures to occupational carcinogens are believed to have such a relevant time window of exposure. Cancer is a disease with a long latency period, and hence it could be that the exposure which is related to the ultimate development of the disease took place many years before the cancer actually manifested itself. This phenomenon is counter-intuitive, since one would have expected that cumulative exposure over a working lifetime would have been the relevant parameter. The exposure at the time of manifestation of disease may not be of particular importance.

The pattern of exposure—continuous exposure, intermittent exposure and exposure with or without sharp peaks—may be relevant as well. Taking exposure patterns into account is important for both epidemiological studies and for environmental measurements which may be used to monitor compliance with health standards or for environmental control as part of control and prevention programmes. For example, if a health effect is caused by peak exposures, such peak levels must be monitorable in order to be controlled. Monitoring which provides data only about long-term average exposures is not useful since the peak excursion values may well be masked by averaging, and certainly cannot be controlled as they occur.

The biologically relevant exposure or dose for a certain endpoint is often not known because the patterns of intake, uptake, distribution and elimination, or the mechanisms of biotransformation, are not understood in sufficient detail. Both the rate at which an agent enters and leaves the body (the kinetics) and the biochemical processes for handling the substance (biotransformation) will help determine the relationships between exposure, dose and effect.

Environmental monitoring is the measurement and assessment of agents at the workplace to evaluate ambient exposure and related health risks. Biological monitoring is the measurement and assessment of workplace agents or their metabolites in tissue, secreta or excreta to evaluate exposure and assess health risks. Sometimes biomarkers, such as DNA-adducts, are used as measures of exposure. Biomarkers may also be indicative of the mechanisms of the disease process itself, but this is a complex subject, which is covered more fully in the chapter Biological Monitoring and later in the discussion here.

A simplification of the basic model in exposure-response modelling is as follows:

exposure uptake distribution,

elimination, transformationtarget dosephysiopathologyeffect

Depending on the agent, exposure-uptake and exposure-intake relationships can be complex. For many gases, simple approximations can be made, based on the concentration of the agent in the air during the course of a working day and on the amount of air that is inhaled. For dust sampling, deposition patterns are also related to particle size. Size considerations may also lead to a more complex relationship. The chapter Respiratory System provides more detail on the aspect of respiratory toxicity.

Exposure and dose assessment are elements of quantitative risk assessment. Health risk assessment methods often form the basis upon which exposure limits are established for emission levels of toxic agents in the air for environmental as well as for occupational standards. Health risk analysis provides an estimate of the probability (risk) of occurrence of specific health effects or an estimate of the number of cases with these health effects. By means of health risk analysis an acceptable concentration of a toxicant in air, water or food can be provided, given an a priori chosen acceptable magnitude of risk. Quantitative risk analysis has found an application in cancer epidemiology, which explains the strong emphasis on retrospective exposure assessment. But applications of more elaborate exposure assessment strategies can be found in both retrospective as well as prospective exposure assessment, and exposure assessment principles have found applications in studies focused on other endpoints as well, such as benign respiratory disease (Wegman et al. 1992; Post et al. 1994). Two directions in research predominate at this moment. One uses dose estimates obtained from exposure monitoring information, and the other relies on biomarkers as measures of exposure.

Exposure Monitoring and Prediction of Dose

Unfortunately, for many exposures few quantitative data are available for predicting the risk for developing a certain endpoint. As early as 1924, Haber postulated that the severity of the health effect (H) is proportional to the product of exposure concentration (X) and time of exposure (T):

H=X x T

Haber’s law, as it is called, formed the basis for development of the concept that time-weighted average (TWA) exposure measurements—that is, measurements taken and averaged over a certain period of time—would be a useful measure for the exposure. This assumption about the adequacy of the time-weighted average has been questioned for many years. In 1952, Adams and co-workers stated that “there is no scientific basis for the use of the time-weighted average to integrate varying exposures …” (in Atherly 1985). The problem is that many relations are more complex than the relationship that Haber’s law represents. There are many examples of agents where the effect is more strongly determined by concentration than by length of time. For example, interesting evidence from laboratory studies has shown that in rats exposed to carbon tetrachloride, the pattern of exposure (continuous versus intermittent and with or without peaks) as well as the dose can modify the observed risk of the rats developing liver enzyme level changes (Bogers et al. 1987). Another example is bio-aerosols, such as α-amylase enzyme, a dough improver, which can cause allergic diseases in people who work in the bakery industry (Houba et al. 1996). It is unknown whether the risk of developing such a disease is mainly determined by peak exposures, average exposure, or cumulative level of exposure. (Wong 1987; Checkoway and Rice 1992). Information on temporal patterns is not available for most agents, especially not for agents that have chronic effects.

The first attempts to model exposure patterns and estimate dose were published in the 1960s and 1970s by Roach (1966; 1977). He showed that the concentration of an agent reaches an equilibrium value at the receptor after an exposure of infinite duration because elimination counterbalances the uptake of the agent. In an eight-hour exposure, a value of 90% of this equilibrium level can be reached if the half-life of the agent at the target organ is smaller than approximately two-and-a-half hours. This illustrates that for agents with a short half-life, the dose at the target organ is determined by an exposure shorter than an eight-hour period. Dose at the target organ is a function of the product of exposure time and concentration for agents with a long half-life. A similar but more elaborate approach has been applied by Rappaport (1985). He showed that intra-day variability in exposure has a limited influence when dealing with agents with long half-lives. He introduced the term dampening at the receptor.

The information presented above has mainly been used to draw conclusions on appropriate averaging times for exposure measurements for compliance purposes. Since Roach’s papers it is common knowledge that for irritants, grab samples with short averaging times have to be taken, while for agents with long half-lives, such as asbestos, long-term average of cumulative exposure has to be approximated. One should however realize that the dichotomization into grab sample strategies and eight-hour time average exposure strategies as adopted in many countries for compliance purposes is an extremely crude translation of the biological principles discussed above.

An example of improving an exposure assessment strategy based on pharmocokinetic principles in epidemiology can be found in a paper of Wegman et al. (1992). They applied an interesting exposure assessment strategy by using continuous monitoring devices to measure personal dust exposure peak levels and relating these to acute reversible respiratory symptoms occurring every 15 minutes.A conceptual problem in this kind of study, extensively discussed in their paper, is the definition of a health-relevant peak exposure. The definition of a peak will, again, depend on biological considerations. Rappaport (1991) gives two requirements for peak exposures to be of aetiological relevance in the disease process: (1) the agent is eliminated rapidly from the body and (2) there is a non-linear rate of biological damage during a peak exposure. Non-linear rates of biological damage may be related to changes in uptake, which in turn are related to exposure levels, host susceptibility, synergy with other exposures, involvement of other disease mechanisms at higher exposures or threshold levels for disease processes.

These examples also show that pharmacokinetic approaches can lead elsewhere than to dose estimates. The results of pharmacokinetic modelling can also be used to explore the biological relevance of existing indices of exposure and to design new health-relevant exposure assessment strategies.

Pharmacokinetic modelling of the exposure may also generate estimates of the actual dose at the target organ. For instance in the case of ozone, an acute irritant gas, models have been developed which predict the tissue concentration in the airways as a function of the average ozone concentration in the airspace of the lung at a certain distance from the trachea, the radius of the airways, the average air velocity, the effective dispersion, and the ozone flux from air to lung surface (Menzel 1987; Miller and Overton 1989). Such models can be used to predict ozone dose in a particular region of the airways, dependent on environmental ozone concentrations and breathing patterns.

In most cases estimates of target dose are based on information on the exposure pattern over time, job history and pharmacokinetic information on uptake, distribution, elimination and transformation of the agent. The whole process can be described by a set of equations which can be mathematically solved. Often information on pharmacokinetic parameters is not available for humans, and parameter estimates based on animal experiments have to be used. There are several examples by now of the use of pharmacokinetic modelling of exposure in order to generate dose estimates. The first references to modelling of exposure data into dose estimates in the literature go back to the paper of Jahr (1974).

Although dose estimates have generally not been validated and have found limited application in epidemiological studies, the new generation of exposure or dose indices is expected to result in optimal exposure-response analyses in epidemiological studies (Smith 1985, 1987). A problem not yet tackled in pharmacokinetic modelling is that large interspecies differences exist in kinetics of toxic agents, and therefore effects of intra-individual variation in pharmacokinetic parameters are of interest (Droz 1992).

Biomonitoring and Biomarkers of Exposure

Biological monitoring offers an estimate of dose and therefore is often considered superior to environmental monitoring. However, the intra-individual variability of biomonitoring indices can be considerable. In order to derive an acceptable estimate of a worker’s dose, repeated measurements have to be taken, and sometimes the measurement effort can become larger than for environmental monitoring.

This is illustrated by an interesting study on workers producing boats made of plastic reinforced with glass fibre (Rappaport et al. 1995). The variability of styrene exposure was assessed by measuring styrene in air repeatedly. Styrene in exhaled air of exposed workers was monitored, as well as sister chromatid exchanges (SCEs). They showed that an epidemiological study using styrene in the air as a measure of exposure would be more efficient, in terms of numbers of measurements required, than a study using the other indices of exposure. For styrene in air three repeats were required to estimate the long-term average exposure with a given precision. For styrene in exhaled air, four repeats per worker were necessary, while for the SCEs 20 repeats were necessary. The explanation for this observation is the signal-to-noise ratio, determined by the day-to-day and between-worker variability in exposure, which was more favourable for styrene in air than for the two biomarkers of exposure. Thus, although the biological relevance of a certain exposure surrogate might be optimal, the performance in an exposure-response analysis can still be poor because of a limited signal-to-noise ratio, leading to misclassification error.

Droz (1991) applied pharmacokinetic modelling to study advantages of exposure assessment strategies based on air sampling compared to biomonitoring strategies dependent on the half-life of the agent. He showed that biological monitoring is also greatly affected by biological variability, which is not related to variability of the toxicological test. He suggested that no statistical advantage exists in using biological indicators when the half-life of the agent considered is smaller than about ten hours.

Although one might tend to decide to measure the environmental exposure instead of a biological indicator of an effect because of variability in the variable measured, additional arguments can be found for choosing a biomarker, even when this would lead to a greater measurement effort, such as when a considerable dermal exposure is present. For agents like pesticides and some organic solvents, dermal exposure can be of greater relevance that the exposure through the air. A biomarker of exposure would include this route of exposure, while measuring of dermal exposure is complex and results are not easily interpretable (Boleij et al. 1995). Early studies among agricultural workers using “pads” to assess dermal exposure showed remarkable distributions of pesticides over the body surface, depending on the tasks of the worker. However, because little information is available on skin uptake, exposure profiles cannot yet be used to estimate an internal dose.

Biomarkers can also have considerable advantages in cancer epidemiology. When a biomarker is an early marker of the effect, its use could result in reduction of the follow-up period. Although validation studies are required, biomarkers of exposure or individual susceptibility could result in more powerful epidemiological studies and more precise risk estimates.

Time Window Analysis

Parallel to the development of pharmacokinetic modelling, epidemiologists have explored new approaches in the data analysis phase such as “time frame analysis” to relate relevant exposure periods to endpoints, and to implement effects of temporal patterns in the exposure or peak exposures in occupational cancer epidemiology (Checkoway and Rice 1992). Conceptually this technique is related to pharmacokinetic modelling since the relationship between exposure and outcome is optimized by putting weights on different exposure periods, exposure patterns and exposure levels. In pharmacokinetic modelling these weights are believed to have a physiological meaning and are estimated beforehand. In time frame analysis the weights are estimated from the data on the basis of statistical criteria. Examples of this approach are given by Hodgson and Jones (1990), who analysed the relationship between radon gas exposure and lung cancer in a cohort of UK tin miners, and by Seixas, Robins and Becker (1993), who analysed the relationship between dust exposure and respiratory health in a cohort of US coal miners. A very interesting study underlining the relevance of time window analysis is the one by Peto et al. (1982).

They showed that mesothelioma death rates appeared to be proportional to some function of time since first exposure and cumulative exposure in a cohort of insulation workers. Time since first exposure was of particular relevance because this variable was an approximation of the time required for a fibre to migrate from its place of deposition in the lungs to the pleura. This example shows how kinetics of deposition and migration determine the risk function to a large extent. A potential problem with time frame analysis is that it requires detailed information on exposure periods and exposure levels, which hampers its application in many studies of chronic disease outcomes.

Concluding Remarks

In conclusion, the underlying principles of pharmacokinetic modelling and time frame or time window analysis are widely recognized. Knowledge in this area has mainly been used to develop exposure assessment strategies. More elaborate use of these approaches, however, requires a considerable research effort and has to be developed. The number of applications is therefore still limited. Relatively simple applications, such as the development of more optimal exposure assessment strategies dependent on the endpoint, have found wider use. An important issue in the development of biomarkers of exposure or effect is validation of these indices. It is often assumed that a measurable biomarker can predict health risk better than traditional methods. However, unfortunately, very few validation studies substantiate this assumption.

 

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Group 1—Carcinogenic to Humans (74)

Agents and groups of agents

Aflatoxins [1402-68-2] (1993)

4-Aminobiphenyl [92-67-1]

Arsenic [7440-38-2] and arsenic compounds2

Asbestos [1332-21-4]

Azathioprine [446-86-6]

Benzene [71-43-2]

Benzidine [92-87-5]

Beryllium [7440-41-7] and beryllium compounds (1993)3

Bis(2-chloroethyl)-2-naphthylamine (Chlornaphazine)[494-03-1]

Bis(chloromethyl)ether [542-88-1] and chloromethyl methyl ether [107-30-2] (technical-grade)

1,4-Butanediol dimethanesulphonate (Myleran) [55-98-1]

Cadmium [7440-43-9] and cadmium compounds (1993)3

Chlorambucil [305-03-3]

1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (Methyl-CCNU; Semustine) [13909-09-6]

Chromium[VI] compounds (1990)3

Ciclosporin [79217-60-0] (1990)

Cyclophosphamide [50-18-0] [6055-19-2]

Diethylstilboestrol [56-53-1]

Erionite [66733-21-9]

Ethylene oxide4 [75-21-8] (1994)

Helicobacter pylori (infection with) (1994)

Hepatitis B virus (chronic infection with) (1993)

Hepatitis C virus (chronic infection with) (1993)

Human papillomavirus type 16 (1995)

Human papillomavirus type 18 (1995)

Human T-cell lymphotropic virus type I (1996)

Melphalan [148-82-3]

8-Methoxypsoralen (Methoxsalen) [298-81-7] plus ultraviolet A radiation

MOPP and other combined chemotherapy including alkylating agents

Mustard gas (Sulphur mustard) [505-60-2]

2-Naphthylamine [91-59-8]

Nickel compounds (1990)3

Oestrogen replacement therapy

Oestrogens, nonsteroidal2

Oestrogens, steroidal2

Opisthorchis viverrini (infection with) (1994)

Oral contraceptives, combined5

Oral contraceptives, sequential

Radon [10043-92-2] and its decay products (1988)

Schistosoma haematobium (infection with) (1994)

Silica [14808-60-7] crystalline (inhaled in the form of quartz or cristobalite from occupational sources)

Solar radiation (1992)

Talc containing asbestiform fibres

Tamoxifen [10540-29-1]6

Thiotepa [52-24-4] (1990)

Treosulphan [299-75-2]

Vinyl chloride [75-01-4]

Mixtures

Alcoholic beverages (1988)

Analgesic mixtures containing phenacetin

Betel quid with tobacco

Coal-tar pitches [65996-93-2]

Coal-tars [8007-45-2]

Mineral oils, untreated and mildly treated

Salted fish (Chinese-style) (1993)

Shale oils [68308-34-9]

Soots

Tobacco products, smokeless

Tobacco smoke

Wood dust

Exposure circumstances

Aluminium production

Auramine, manufacture of

Boot and shoe manufacture and repair

Coal gasification

Coke production

Furniture and cabinet making

Haematite mining (underground) with exposure to radon

Iron and steel founding

Isopropanol manufacture (strong-acid process)

Magenta, manufacture of (1993)

Painter (occupational exposure as a) (1989)

Rubber industry

Strong-inorganic-acid mists containing sulphuric acid (occupational exposure to) (1992)

Group 2A—Probably carcinogenic to humans (56)

Agents and groups of agents

Acrylamide [79-06-1] (1994)8

Acrylonitrile [107-13-1]

Adriamycin8 [23214-92-8]

Androgenic (anabolic) steroids

Azacitidine8 [320-67-2] (1990)

Benz[a]anthracene8 [56-55-3]

Benzidine-based dyes8

Benzo[a]pyrene8 [50-32-8]

Bischloroethyl nitrosourea (BCNU) [154-93-8]

1,3-Butadiene [106-99-0] (1992)

Captafol [2425-06-1] (1991)

Chloramphenicol [56-75-7] (1990)

1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea8 (CCNU)[13010-47-4]

p-Chloro-o-toluidine [95-69-2] and its strong acid salts (1990)3

Chlorozotocin8 [54749-90-5] (1990)

Cisplatin8 [15663-27-1]

Clonorchis sinensis (infection with)8 (1994)

Dibenz[a,h]anthracene8 [53-70-3]

Diethyl sulphate [64-67-5] (1992)

Dimethylcarbamoyl chloride8 [79-44-7]

Dimethyl sulphate8 [77-78-1]

Epichlorohydrin8 [106-89-8]

Ethylene dibromide8 [106-93-4]

N-Ethyl-N-nitrosourea8 [759-73-9]

Formaldehyde [50-00-0])

IQ8 (2-Amino-3-methylimidazo[4,5-f]quinoline) [76180-96-6] (1993)

5-Methoxypsoralen8 [484-20-8]

4,4´-Methylene bis(2-chloroaniline) (MOCA)8 [101-14-4] (1993)

N-Methyl-N´-nitro-N-nitrosoguanidine8 (MNNG) [70-25-7]

N-Methyl-N-nitrosourea8 [684-93-5]

Nitrogen mustard [51-75-2]

N-Nitrosodiethylamine8 [55-18-5]

N-Nitrosodimethylamine 8 [62-75-9]

Phenacetin [62-44-2]

Procarbazine hydrochloride8 [366-70-1]

Tetrachloroethylene [127-18-4]

Trichloroethylene [79-01-6]

Styrene-7,8-oxide8 [96-09-3] (1994)

Tris(2,3-dibromopropyl)phosphate8 [126-72-7]

Ultraviolet radiation A8 (1992)

Ultraviolet radiation B8 (1992)

Ultraviolet radiation C8 (1992)

Vinyl bromide6 [593-60-2]

Vinyl fluoride [75-02-5]

Mixtures

Creosotes [8001-58-9]

Diesel engine exhaust (1989)

Hot mate (1991)

Non-arsenical insecticides (occupational exposures in spraying and application of) (1991)

Polychlorinated biphenyls [1336-36-3]

Exposure circumstances

Art glass, glass containers and pressed ware (manufacture of) (1993)

Hairdresser or barber (occupational exposure as a) (1993)

Petroleum refining (occupational exposures in) (1989)

Sunlamps and sunbeds (use of) (1992)

Group 2B—Possibly carcinogenic to humans (225)

Agents and groups of agents

A–α–C (2-Amino-9H-pyrido[2,3-b]indole) [26148-68-5]

Acetaldehyde [75-07-0]

Acetamide [60-35-5]

AF-2 [2-(2-Furyl)-3-(5-nitro-2-furyl)acrylamide] [3688-53-7]

Aflatoxin M1 [6795-23-9] (1993)

p-Aminoazobenzene [60-09-3]

o-Aminoazotoluene [97-56-3]

2-Amino-5-(5-nitro-2-furyl)-1,3,4-thiadiazole [712-68-5]

Amitrole [61-82-5]

o-Anisidine [90-04-0]

Antimony trioxide [1309-64-4] (1989)

Aramite [140-57-8]

Atrazine9 [1912-24-9] (1991)

Auramine [492-80-8] (technical-grade)

Azaserine [115-02-6]

Benzo[b]fluoranthene [205-99-2]

Benzo[j]fluoranthene [205-82-3]

Benzo[k]fluoranthene [207-08-9]

Benzyl violet 4B [1694-09-3]

Bleomycins [11056-06-7]

Bracken fern

Bromodichloromethane [75-27-4] (1991)

Butylated hydroxyanisole (BHA) [25013-16-5]

β-Butyrolactone [3068-88-0]

Caffeic acid [331-39-5] (1993)

Carbon-black extracts

Carbon tetrachloride [56-23-5]

Ceramic fibres

Chlordane [57-74-9] (1991)

Chlordecone (Kepone) [143-50-0]

Chlorendic acid [115-28-6] (1990)

α-Chlorinated toluenes (benzyl chloride, benzal chloride,benzotrichloride)

p-Chloroaniline [106-47-8] (1993)

Chloroform [67-66-3]

1-Chloro-2-methylpropene [513-37-1]

Chlorophenols

Chlorophenoxy herbicides

4-Chloro-o-phenylenediamine [95-83-0]

CI Acid Red 114 [6459-94-5] (1993)

CI Basic Red 9 [569-61-9] (1993)

CI Direct Blue 15 [2429-74-5] (1993)

Citrus Red No. 2 [6358-53-8]

Cobalt [7440-48-4] and cobalt compounds3 (1991)

p-Cresidine [120-71-8]

Cycasin [14901-08-7]

Dacarbazine [4342-03-4]

Dantron (Chrysazin; 1,8-Dihydroxyanthraquinone) [117-10-2] (1990)

Daunomycin [20830-81-3]

DDT´-DDT, 50-29-3] (1991)

N,N´-Diacetylbenzidine [613-35-4]

2,4-Diaminoanisole [615-05-4]

4,4´-Diaminodiphenyl ether [101-80-4]

2,4-Diaminotoluene [95-80-7]

Dibenz[a,h]acridine [226-36-8]

Dibenz[a,j]acridine [224-42-0]

7H-Dibenzo[c,g]carbazole [194-59-2]

Dibenzo[a,e]pyrene [192-65-4]

Dibenzo[a,h]pyrene [189-64-0]

Dibenzo[a,i]pyrene [189-55-9]

Dibenzo[a,l]pyrene [191-30-0]

1,2-Dibromo-3-chloropropane [96-12-8]

p-Dichlorobenzene [106-46-7]

3,3´-Dichlorobenzidine [91-94-1]

3,3´-Dichloro-4,4´-diaminodiphenyl ether [28434-86-8]

1,2-Dichloroethane [107-06-2]

Dichloromethane (methylene chloride) [75-09-2]

1,3-Dichloropropene [542-75-6] (technical grade)

Dichlorvos [62-73-7] (1991)

Diepoxybutane [1464-53-5]

Di(2-ethylhexyl)phthalate [117-81-7]

1,2-Diethylhydrazine [1615-80-1]

Diglycidyl resorcinol ether [101-90-6]

Dihydrosafrole [94-58-6]

Diisopropyl sulphate [2973-10-6] (1992)

3,3´-Dimethoxybenzidine (o-Dianisidine) [119-90-4]

p-Dimethylaminoazobenzene [60-11-7]

trans-2-[(Dimethylamino)methylimino]-5-[2-(5-nitro-2-furyl)-vinyl]-1,3,4-oxadiazole [25962-77-0]

2,6-Dimethylaniline (2,6-xylidine) [87-62-7] (1993)

3,3´-Dimethylbenzidine (o-tolidine) [119-93-7]

Dimethylformamide [68-12-2] (1989)

1,1-Dimethylhydrazine [57-14-7]

1,2-Dimethylhydrazine [540-73-8]

3,7-Dinitrofluoranthene [105735-71-5]

3,9-Dinitrofluoranthene [22506-53-2]

1,6-Dinitropyrene [42397-64-8] (1989)

1,8-Dinitropyrene [42397-65-9] (1989)

2,4-Dinitrotoluene [121-14-2]

2,6-Dinitrotoluene [606-20-2]

1,4-Dioxane [123-91-1]

Disperse Blue 1 [2475-45-8] (1990)

Ethyl acrylate [140-88-5]

Ethylene thiourea [96-45-7]

Ethyl methanesulphonate [62-50-0]

2-(2-Formylhydrazino)-4-(5-nitro-2-furyl)thiazole [3570-75-0]

Glass wool (1988)

Glu-P-1 (2-amino-6-methyldipyrido[1,2-a:3´,2´-d]imidazole)[67730-11-4]

Glu-P-2 (2-aminodipyrido[1,2-a:3´,2´-d]imidazole) [67730-10-3]

Glycidaldehyde [765-34-4]

Griseofulvin [126-07-8]

HC Blue No. 1 [2784-94-3] (1993)

Heptachlor [76-44-8] (1991)

Hexachlorobenzene [118-74-1]

Hexachlorocyclohexanes

Hexamethylphosphoramide [680-31-9]

Human immunodeficiency virus type 2 (infection with) (1996)

Human papillomaviruses: some types other than 16, 18, 31 and 33 (1995)

Hydrazine [302-01-2]

Indeno[1,2,3-cd]pyrene [193-39-5]

Iron-dextran complex [9004-66-4]

Isoprene [78-79-5] (1994)

Lasiocarpine [303-34-4]

Lead [7439-92-1] and lead compounds, inorganic3

Magenta [632-99-5] (containing CI Basic Red 9) (1993)

MeA-α-C (2-Amino-3-methyl-9H-pyrido[2,3-b]indole)[68006-83-7]

Medroxyprogesterone acetate [71-58-9]

MeIQ (2-Amino-3,4-dimethylimidazo[4,5-f]quinoline)[77094-11-2] (1993)

MeIQx (2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline) [77500-04-0] (1993)

Merphalan [531-76-0]

2-Methylaziridine (propyleneimine) [75-55-8]

Methylazoxymethanol acetate [592-62-1]

5-Methylchrysene [3697-24-3]

4,4´-Methylene bis(2-methylaniline) [838-88-0]

4,4´-Methylenedianiline [101-77-9]

Methylmercury compounds (1993)3

Methyl methanesulphonate [66-27-3]

2-Methyl-1-nitroanthraquinone [129-15-7] (uncertain purity)

N-Methyl-N-nitrosourethane [615-53-2]

Methylthiouracil [56-04-2]

Metronidazole [443-48-1]

Mirex [2385-85-5]

Mitomycin C [50-07-7]

Monocrotaline [315-22-0]

5-(Morpholinomethyl)-3-[(5-nitrofurfurylidene)amino]-2-oxazolidinone [3795-88-8]

Nafenopin [3771-19-5]

Nickel, metallic [7440-02-0] (1990)

Niridazole [61-57-4]

Nitrilotriacetic acid [139-13-9] and its salts (1990)3

5-Nitroacenaphthene [602-87-9]

2-Nitroanisole [91-23-6] (1996)

Nitrobenzene [98-95-3] (1996)

6-Nitrochrysene [7496-02-8] (1989)

Nitrofen [1836-75-5], technical-grade

2-Nitrofluorene [607-57-8] (1989)

1-[(5-Nitrofurfurylidene)amino]-2-imidazolidinone [555-84-0]

N-[4-(5-Nitro-2-furyl)-2-thiazolyl]acetamide [531-82-8]

Nitrogen mustard N-oxide [126-85-2]

2-Nitropropane [79-46-9]

1-Nitropyrene [5522-43-0] (1989)

4-Nitropyrene [57835-92-4] (1989)

N-Nitrosodi-n-butylamine [924-16-3]

N-Nitrosodiethanolamine [1116-54-7]

N-Nitrosodi-n-propylamine [621-64-7]

3-(N-Nitrosomethylamino)propionitrile [60153-49-3]

4-(N-Nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) [64091-91-4]

N-Nitrosomethylethylamine [10595-95-6]

N-Nitrosomethylvinylamine [4549-40-0]

N-Nitrosomorpholine [59-89-2]

N‘-Nitrosonornicotine [16543-55-8]

N-Nitrosopiperidine [100-75-4]

N-Nitrosopyrrolidine [930-55-2]

N-Nitrososarcosine [13256-22-9]

Ochratoxin A [303-47-9] (1993)

Oil Orange SS [2646-17-5]

Oxazepam [604-75-1] (1996)

Palygorskite (attapulgite) [12174-11-7] (long fibres, >>5 micro-meters) (1997)

Panfuran S (containing dihydroxymethylfuratrizine [794-93-4])

Pentachlorophenol [87-86-5] (1991)

Phenazopyridine hydrochloride [136-40-3]

Phenobarbital [50-06-6]

Phenoxybenzamine hydrochloride [63-92-3]

Phenyl glycidyl ether [122-60-1] (1989)

Phenytoin [57-41-0]

PhIP (2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) [105650-23-5] (1993)

Ponceau MX [3761-53-3]

Ponceau 3R [3564-09-8]

Potassium bromate [7758-01-2]

Progestins

1,3-Propane sultone [1120-71-4]

β-Propiolactone [57-57-8]

Propylene oxide [75-56-9] (1994)

Propylthiouracil [51-52-5]

Rockwool (1988)

Saccharin [81-07-2]

Safrole [94-59-7]

Schistosoma japonicum (infection with) (1994)

Slagwool (1988)

Sodium o-phenylphenate [132-27-4]

Sterigmatocystin [10048-13-2]

Streptozotocin [18883-66-4]

Styrene [100-42-5] (1994)

Sulfallate [95-06-7]

Tetranitromethane [509-14-8] (1996)

Thioacetamide [62-55-5]

4,4´-Thiodianiline [139-65-1]

Thiourea [62-56-6]

Toluene diisocyanates [26471-62-5]

o-Toluidine [95-53-4]

Trichlormethine (Trimustine hydrochloride) [817-09-4] (1990)

Trp-P-1 (3-Amino-1,4-dimethyl-5H-pyrido[4,3-b]indole) [62450-06-0]

Trp-P-2 (3-Amino-1-methyl-5H-pyrido[4,3-b]indole) [62450-07-1]

Trypan blue [72-57-1]

Uracil mustard [66-75-1]

Urethane [51-79-6]

Vinyl acetate [108-05-4] (1995)

4-Vinylcyclohexene [100-40-3] (1994)

4-Vinylcyclohexene diepoxide [107-87-6] (1994)

Mixtures

Bitumens [8052-42-4], extracts of steam-refined and air-refined

Carrageenan [9000-07-1], degraded

Chlorinated paraffins of average carbon chain length C12 and average degree of chlorination approximately 60% (1990)

Coffee (urinary bladder)9 (1991)

Diesel fuel, marine (1989)

Engine exhaust, gasoline (1989)

Fuel oils, residual (heavy) (1989)

Gasoline (1989)

Pickled vegetables (traditional in Asia) (1993)

Polybrominated biphenyls [Firemaster BP-6, 59536-65-1]

Toxaphene (Polychlorinated camphenes) [8001-35-2]

Toxins derived from Fusarium moniliforme (1993)

Welding fumes (1990)

Exposure circumstances

Carpentry and joinery

Dry cleaning (occupational exposures in) (1995)

Printing processes (occupational exposures in) (1996)

Textile manufacturing industry (work in) (1990)

Group 3—Unclassifiable as to carcinogenicity to humans (480)

Agents and groups of agents

Acridine orange [494-38-2]

Acriflavinium chloride [8018-07-3]

Acrolein [107-02-8]

Acrylic acid [79-10-7]

Acrylic fibres

Acrylonitrile-butadiene-styrene copolymers

Actinomycin D [50-76-0]

Aldicarb [116-06-3] (1991)

Aldrin [309-00-2]

Allyl chloride [107-05-1]

Allyl isothiocyanate [57-06-7]

Allyl isovalerate [2835-39-4]

Amaranth [915-67-3]

5-Aminoacenaphthene [4657-93-6]

2-Aminoanthraquinone [117-79-3]

p-Aminobenzoic acid [150-13-0]

1-Amino-2-methylanthraquinone [82-28-0]

2-Amino-4-nitrophenol [99-57-0] (1993)

2-Amino-5-nitrophenol [121-88-0] (1993)

4-Amino-2-nitrophenol [119-34-6]

2-Amino-5-nitrothiazole [121-66-4]

11-Aminoundecanoic acid [2432-99-7]

Ampicillin [69-53-4] (1990)

Anaesthetics, volatile

Angelicin [523-50-2] plus ultraviolet A radiation

Aniline [62-53-3]

p-Anisidine [104-94-9]

Anthanthrene [191-26-4]

Anthracene [120-12-7]

Anthranilic acid [118-92-3]

Antimony trisulphide [1345-04-6] (1989)

Apholate [52-46-0]

p-Aramid fibrils [24938-64-5] (1997)

Aurothioglucose [12192-57-3]

Aziridine [151-56-4]

2-(1-Aziridinyl)ethanol [1072-52-2]

Aziridyl benzoquinone [800-24-8]

Azobenzene [103-33-3]

Benz[a]acridine [225-11-6]

Benz[c]acridine [225-51-4]

Benzo[ghi]fluoranthene [203-12-3]

Benzo[a]fluorene [238-84-6]

Benzo[b]fluorene [243-17-4]

Benzo[c]fluorene [205-12-9]

Benzo[ghi]perylene [191-24-2]

Benzo[c]phenanthrene [195-19-7]

Benzo[e]pyrene [192-97-2]

p-Benzoquinone dioxime [105-11-3]

Benzoyl chloride [98-88-4]

Benzoyl peroxide [94-36-0]

Benzyl acetate [140-11-4]

Bis(1-aziridinyl)morpholinophosphine sulphide [2168-68-5]

Bis(2-chloroethyl)ether [111-44-4]

1,2-Bis(chloromethoxy)ethane [13483-18-6]

1,4-Bis(chloromethoxymethyl)benzene [56894-91-8]

Bis(2-chloro-1-methylethyl)ether [108-60-1]

Bis(2,3-epoxycyclopentyl)ether [2386-90-5] (1989)

Bisphenol A diglycidyl ether [1675-54-3] (1989)

Bisulphites (1992)

Blue VRS [129-17-9]

Brilliant Blue FCF, disodium salt [3844-45-9]

Bromochloroacetonitrile [83463-62-1] (1991)

Bromoethane [74-96-4] (1991)

Bromoform [75-25-2] (1991)

n-Butyl acrylate [141-32-2]

Butylated hydroxytoluene (BHT) [128-37-0]

Butyl benzyl phthalate [85-68-7]

γ-Butyrolactone [96-48-0]

Caffeine [58-08-2] (1991)

Cantharidin [56-25-7]

Captan [133-06-2]

Carbaryl [63-25-2]

Carbazole [86-74-8]

3-Carbethoxypsoralen [20073-24-9]

Carmoisine [3567-69-9]

Carrageenan [9000-07-1], native

Catechol [120-80-9]

Chloral [75-87-6] (1995)

Chloral hydrate [302-17-0] (1995)

Chlordimeform [6164-98-3]

Chlorinated dibenzodioxins (other than TCDD)

Chlorinated drinking-water (1991)

Chloroacetonitrile [107-14-2] (1991)

Chlorobenzilate [510-15-6]

Chlorodibromomethane [124-48-1] (1991)

Chlorodifluoromethane [75-45-6]

Chloroethane [75-00-3] (1991)

Chlorofluoromethane [593-70-4]

3-Chloro-2-methylpropene [563-47-3] (1995)

4-Chloro-m-phenylenediamine [5131-60-2]

Chloronitrobenzenes [88-73-3; 121-73-3; 100-00-5] (1996)

Chloroprene [126-99-8]

Chloropropham [101-21-3]

Chloroquine [54-05-7]

Chlorothalonil [1897-45-6]

2-Chloro-1,1,1-trifluoroethane [75-88-7]

Cholesterol [57-88-5]

Chromium[III] compounds (1990)

Chromium [7440-47-3], metallic (1990)

Chrysene [218-01-9]

Chrysoidine [532-82-1]

CI Acid Orange 3 [6373-74-6] (1993)

Cimetidine [51481-61-9] (1990)

Cinnamyl anthranilate [87-29-6]

CI Pigment Red 3 [2425-85-6] (1993)

Citrinin [518-75-2]

Clofibrate [637-07-0]

Clomiphene citrate [50-41-9]

Coal dust (1997)

Copper 8-hydroxyquinoline [10380-28-6]

Coronene [191-07-1]

Coumarin [91-64-5]

m-Cresidine [102-50-1]

Crotonaldehyde [4170-30-3] (1995)

Cyclamates [sodium cyclamate, 139-05-9]

Cyclochlorotine [12663-46-6]

Cyclohexanone [108-94-1] (1989)

Cyclopenta[cd]pyrene [27208-37-3]

D & C Red No. 9 [5160-02-1] (1993)

Dapsone [80-08-0]

Decabromodiphenyl oxide [1163-19-5] (1990)

Deltamethrin [52918-63-5] (1991)

Diacetylaminoazotoluene [83-63-6]

Diallate [2303-16-4]

1,2-Diamino-4-nitrobenzene [99-56-9]

1,4-Diamino-2-nitrobenzene [5307-14-2] (1993)

2,5-Diaminotoluene [95-70-5]

Diazepam [439-14-5]

Diazomethane [334-88-3]

Dibenz[a,c]anthracene [215-58-7]

Dibenz[a,j]anthracene [224-41-9]

Dibenzo-p-dioxin (1997)

Dibenzo[a,e]fluoranthene [5385-75-1]

Dibenzo[h,rst]pentaphene [192-47-2]

Dibromoacetonitrile [3252-43-5] (1991)

Dichloroacetic acid [79-43-6] (1995)

Dichloroacetonitrile [3018-12-0] (1991)

Dichloroacetylene [7572-29-4]

o-Dichlorobenzene [95-50-1]

trans-1,4-Dichlorobutene [110-57-6]

2,6-Dichloro-para-phenylenediamine [609-20-1]

1,2-Dichloropropane [78-87-5]

Dicofol [115-32-2]

Dieldrin [60-57-1]

Di(2-ethylhexyl)adipate [103-23-1]

Dihydroxymethylfuratrizine [794-93-4]

Dimethoxane [828-00-2]

3,3´-Dimethoxybenzidine-4,4´-diisocyanate [91-93-0]

p-Dimethylaminoazobenzenediazo sodium sulphonate[140-56-7]

4,4´-Dimethylangelicin [22975-76-4] plus ultraviolet Aradiation

4,5´-Dimethylangelicin [4063-41-6] plus ultraviolet A

N,N-Dimethylaniline [121-69-7] (1993)

Dimethyl hydrogen phosphite [868-85-9] (1990)

1,4-Dimethylphenanthrene [22349-59-3]

1,3-Dinitropyrene [75321-20-9] (1989)

Dinitrosopentamethylenetetramine [101-25-7]

2,4´-Diphenyldiamine [492-17-1]

Disperse Yellow 3 [2832-40-8] (1990)

Disulfiram [97-77-8]

Dithranol [1143-38-0]

Doxefazepam [40762-15-0] (1996)

Droloxifene [82413-20-5] (1996)

Dulcin [150-69-6]

Endrin [72-20-8]

Eosin [15086-94-9]

1,2-Epoxybutane [106-88-7] (1989)

3,4-Epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexane carboxylate [141-37-7]

cis-9,10-Epoxystearic acid [2443-39-2]

Estazolam [29975-16-4] (1996)

Ethionamide [536-33-4]

Ethylene [74-85-1] (1994)

Ethylene sulphide [420-12-2]

2-Ethylhexyl acrylate [103-11-7] (1994)

Ethyl selenac [5456-28-0]

Ethyl tellurac [20941-65-5]

Eugenol [97-53-0]

Evans blue [314-13-6]

Fast Green FCF [2353-45-9]

Fenvalerate [51630-58-1] (1991)

Ferbam [14484-64-1]

Ferric oxide [1309-37-1]

Fluometuron [2164-17-2]

Fluoranthene [206-44-0]

Fluorene [86-73-7]

Fluorescent lighting (1992)

Fluorides (inorganic, used in drinking-water)

5-Fluorouracil [51-21-8]

Furazolidone [67-45-8]

Furfural [98-01-1] (1995)

Furosemide (Frusemide) [54-31-9] (1990)

Gemfibrozil [25812-30-0] (1996)

Glass filaments (1988)

Glycidyl oleate [5431-33-4]

Glycidyl stearate [7460-84-6]

Guinea Green B [4680-78-8]

Gyromitrin [16568-02-8]

Haematite [1317-60-8]

HC Blue No. 2 [33229-34-4] (1993)

HC Red No. 3 [2871-01-4] (1993)

HC Yellow No. 4 [59820-43-8] (1993)

Hepatitis D virus (1993)

Hexachlorobutadiene [87-68-3]

Hexachloroethane [67-72-1]

Hexachlorophene [70-30-4]

Human T-cell lymphotropic virus type II (1996)

Hycanthone mesylate [23255-93-8]

Hydralazine [86-54-4]

Hydrochloric acid [7647-01-0] (1992)

Hydrochlorothiazide [58-93-5] (1990)

Hydrogen peroxide [7722-84-1]

Hydroquinone [123-31-9]

4-Hydroxyazobenzene [1689-82-3]

8-Hydroxyquinoline [148-24-3]

Hydroxysenkirkine [26782-43-4]

Hypochlorite salts (1991)

Iron-dextrin complex [9004-51-7]

Iron sorbitol-citric acid complex [1338-16-5]

Isatidine [15503-86-3]

Isonicotinic acid hydrazide (Isoniazid) [54-85-3]

Isophosphamide [3778-73-2]

Isopropanol [67-63-0]

Isopropyl oils

Isosafrole [120-58-1]

Jacobine [6870-67-3]

Kaempferol [520-18-3]

Lauroyl peroxide [105-74-8]

Lead, organo [75-74-1], [78-00-2]

Light Green SF [5141-20-8]

d-Limonene [5989-27-5] (1993)

Luteoskyrin [21884-44-6]

Malathion [121-75-5]

Maleic hydrazide [123-33-1]

Malonaldehyde [542-78-9]

Maneb [12427-38-2]

Mannomustine dihydrochloride [551-74-6]

Medphalan [13045-94-8]

Melamine [108-78-1]

6-Mercaptopurine [50-44-2]

Mercury [7439-97-6] and inorganic mercury compounds (1993)

Metabisulphites (1992)

Methotrexate [59-05-2]

Methoxychlor [72-43-5]

Methyl acrylate [96-33-3]

5-Methylangelicin [73459-03-7] plus ultraviolet A radiation

Methyl bromide [74-83-9]

Methyl carbamate [598-55-0]

Methyl chloride [74-87-3]

1-Methylchrysene [3351-28-8]

2-Methylchrysene [3351-32-4]

3-Methylchrysene [3351-31-3]

4-Methylchrysene [3351-30-2]

6-Methylchrysene [1705-85-7]

N-Methyl-N,4-dinitrosoaniline [99-80-9]

4,4´-Methylenebis(N,N-dimethyl)benzenamine [101-61-1]

4,4´-Methylenediphenyl diisocyanate [101-68-8]

2-Methylfluoranthene [33543-31-6]

3-Methylfluoranthene [1706-01-0]

Methylglyoxal [78-98-8] (1991)

Methyl iodide [74-88-4]

Methyl methacrylate [80-62-6] (1994)

N-Methylolacrylamide [90456-67-0] (1994)

Methyl parathion [298-00-0]

1-Methylphenanthrene [832-69-9]

7-Methylpyrido[3,4-c]psoralen [85878-62-2]

Methyl red [493-52-7]

Methyl selenac [144-34-3]

Modacrylic fibres

Monuron [150-68-5] (1991)

Morpholine [110-91-8] (1989)

Musk ambrette [83-66-9] (1996)

Musk xylene [81-15-2] (1996)

1,5-Naphthalenediamine [2243-62-1]

1,5-Naphthalene diisocyanate [3173-72-6]

1-Naphthylamine [134-32-7]

1-Naphthylthiourea (ANTU) [86-88-4]

Nithiazide [139-94-6]

5-Nitro-o-anisidine [99-59-2]

9-Nitroanthracene [602-60-8]

7-Nitrobenz[a]anthracene [20268-51-3] (1989

6-Nitrobenzo[a]pyrene [63041-90-7] (1989)

4-Nitrobiphenyl [92-93-3]

3-Nitrofluoranthene [892-21-7]

Nitrofural (Nitrofurazone) [59-87-0] (1990)

Nitrofurantoin [67-20-9] (1990)

1-Nitronaphthalene [86-57-7] (1989)

2-Nitronaphthalene [581-89-5] (1989)

3-Nitroperylene [20589-63-3] (1989)

2-Nitropyrene [789-07-1] (1989)

N´-Nitrosoanabasine [37620-20-5]

N-Nitrosoanatabine [71267-22-6]

N-Nitrosodiphenylamine [86-30-6]

p-Nitrosodiphenylamine [156-10-5]

N-Nitrosofolic acid [29291-35-8]

N-Nitrosoguvacine [55557-01-2]

N-Nitrosoguvacoline [55557-02-3]

N-Nitrosohydroxyproline [30310-80-6]

3-(N-Nitrosomethylamino)propionaldehyde [85502-23-4]

4-(N-Nitrosomethylamino)-4-(3-pyridyl)-1-butanal (NNA) [64091-90-3]

N-Nitrosoproline [7519-36-0]

5-Nitro-o-toluidine [99-55-8] (1990)

Nitrovin [804-36-4]

Nylon 6 [25038-54-4]

Oestradiol mustard [22966-79-6]

Oestrogen-progestin replacement therapy

Opisthorchis felineus (infection with) (1994)

Orange I [523-44-4]

Orange G [1936-15-8]

Oxyphenbutazone [129-20-4]

Palygorskite (attapulgite) [12174-11-7] (short fibres, <<5 micro-meters) (1997)

Paracetamol (Acetaminophen) [103-90-2] (1990)

Parasorbic acid [10048-32-5]

Parathion [56-38-2]

Patulin [149-29-1]

Penicillic acid [90-65-3]

Pentachloroethane [76-01-7]

Permethrin [52645-53-1] (1991)

Perylene [198-55-0]

Petasitenine [60102-37-6]

Phenanthrene [85-01-8]

Phenelzine sulphate [156-51-4]

Phenicarbazide [103-03-7]

Phenol [108-95-2] (1989)

Phenylbutazone [50-33-9]

m-Phenylenediamine [108-45-2]

p-Phenylenediamine [106-50-3]

N-Phenyl-2-naphthylamine [135-88-6]

o-Phenylphenol [90-43-7]

Picloram [1918-02-1] (1991)

Piperonyl butoxide [51-03-6]

Polyacrylic acid [9003-01-4]

Polychlorinated dibenzo-p-dioxins (other than 2,3,7,8-tetra-chlorodibenzo-p-dioxin) (1997)

Polychlorinated dibenzofurans (1997)

Polychloroprene [9010-98-4]

Polyethylene [9002-88-4]

Polymethylene polyphenyl isocyanate [9016-87-9]

Polymethyl methacrylate [9011-14-7]

Polypropylene [9003-07-0]

Polystyrene [9003-53-6]

Polytetrafluoroethylene [9002-84-0]

Polyurethane foams [9009-54-5]

Polyvinyl acetate [9003-20-7]

Polyvinyl alcohol [9002-89-5]

Polyvinyl chloride [9002-86-2]

Polyvinyl pyrrolidone [9003-39-8]

Ponceau SX [4548-53-2]

Potassium bis(2-hydroxyethyl)dithiocarbamate[23746-34-1]

Prazepam [2955-38-6] (1996)

Prednimustine [29069-24-7] (1990)

Prednisone [53-03-2]

Proflavine salts

Pronetalol hydrochloride [51-02-5]

Propham [122-42-9]

n-Propyl carbamate [627-12-3]

Propylene [115-07-1] (1994)

Ptaquiloside [87625-62-5]

Pyrene [129-00-0]

Pyrido[3,4-c]psoralen [85878-62-2]

Pyrimethamine [58-14-0]

Quercetin [117-39-5]

p-Quinone [106-51-4]

Quintozene (Pentachloronitrobenzene) [82-68-8]

Reserpine [50-55-5]

Resorcinol [108-46-3]

Retrorsine [480-54-6]

Rhodamine B [81-88-9]

Rhodamine 6G [989-38-8]

Riddelliine [23246-96-0]

Rifampicin [13292-46-1]

Ripazepam [26308-28-1] (1996)

Rugulosin [23537-16-8]

Saccharated iron oxide [8047-67-4]

Scarlet Red [85-83-6]

Schistosoma mansoni (infection with) (1994)

Selenium [7782-49-2] and selenium compounds

Semicarbazide hydrochloride [563-41-7]

Seneciphylline [480-81-9]

Senkirkine [2318-18-5]

Sepiolite [15501-74-3]

Shikimic acid [138-59-0]

Silica [7631-86-9], amorphous

Simazine [122-34-9] (1991)

Sodium chlorite [7758-19-2] (1991)

Sodium diethyldithiocarbamate [148-18-5]

Spironolactone [52-01-7]

Styrene-acrylonitrile copolymers [9003-54-7]

Styrene-butadiene copolymers [9003-55-8]

Succinic anhydride [108-30-5]

Sudan I [842-07-9]

Sudan II [3118-97-6]

Sudan III [85-86-9]

Sudan Brown RR [6416-57-5]

Sudan Red 7B [6368-72-5]

Sulphafurazole (Sulphisoxazole) [127-69-5]

Sulphamethoxazole [723-46-6]

Sulphites (1992)

Sulphur dioxide [7446-09-5] (1992)

Sunset Yellow FCF [2783-94-0]

Symphytine [22571-95-5]

Talc [14807-96-6], not containing asbestiform fibres

Tannic acid [1401-55-4] and tannins

Temazepam [846-50-4] (1996)

2,2´,5,5´-Tetrachlorobenzidine [15721-02-5]

1,1,1,2-Tetrachloroethane [630-20-6]

1,1,2,2-Tetrachloroethane [79-34-5]

Tetrachlorvinphos [22248-79-9]

Tetrafluoroethylene [116-14-3]

Tetrakis(hydroxymethyl)phosphonium salts (1990)

Theobromine [83-67-0] (1991)

Theophylline [58-55-9] (1991)

Thiouracil [141-90-2]

Thiram [137-26-8] (1991)

Titanium dioxide [13463-67-7] (1989)

Toluene [108-88-3] (1989)

Toremifene [89778-26-7] (1996)

Toxins derived from Fusarium graminearum, F. culmorum andF. crookwellense (1993)

Toxins derived from Fusarium sporotrichioides (1993)

Trichlorfon [52-68-6]

Trichloroacetic acid [76-03-9] (1995)

Trichloroacetonitrile [545-06-2] (1991)

1,1,1-Trichloroethane [71-55-6]

1,1,2-Trichloroethane [79-00-5] (1991)

Triethylene glycol diglydicyl ether [1954-28-5]

Trifluralin [1582-09-8] (1991)

4,4´,6-Trimethylangelicin [90370-29-9] plus ultravioletradiation

2,4,5-Trimethylaniline [137-17-7]

2,4,6-Trimethylaniline [88-05-1]

4,5´,8-Trimethylpsoralen [3902-71-4]

2,4,6-Trinitrotoluene [118-96-7] (1996)

Triphenylene [217-59-4]

Tris(aziridinyl)-p-benzoquinone (Triaziquone) [68-76-8]

Tris(1-aziridinyl)phosphine oxide [545-55-1]

2,4,6-Tris(1-aziridinyl)-s-triazine [51-18-3]

Tris(2-chloroethyl)phosphate [115-96-8] (1990)

1,2,3-Tris(chloromethoxy)propane [38571-73-2]

Tris(2-methyl-1-aziridinyl)phosphine oxide [57-39-6]

Vat Yellow 4 [128-66-5] (1990)

Vinblastine sulphate [143-67-9]

Vincristine sulphate [2068-78-2]

Vinyl acetate [108-05-4]

Vinyl chloride-vinyl acetate copolymers [9003-22-9]

Vinylidene chloride [75-35-4]

Vinylidene chloride-vinyl chloride copolymers [9011-06-7]

Vinylidene fluoride [75-38-7]

N-Vinyl-2-pyrrolidone [88-12-0]

Vinyl toluene [25013-15-4] (1994)

Wollastonite [13983-17-0]

Xylene [1330-20-7] (1989)

2,4-Xylidine [95-68-1]

2,5-Xylidine [95-78-3]

Yellow AB [85-84-7]

Yellow OB [131-79-3]

Zectran [315-18-4]

Zeolites [1318-02-1] other than erionite (clinoptilolite,phillipsite, mordenite, non-fibrous Japanese zeolite,synthetic zeolites) (1997)

Zineb [12122-67-7]

Ziram [137-30-4] (1991)

Mixtures

Betel quid, without tobacco

Bitumens [8052-42-4], steam-refined, cracking-residue and air-refined

Crude oil [8002-05-9] (1989)

Diesel fuels, distillate (light) (1989)

Fuel oils, distillate (light) (1989)

Jet fuel (1989)

Mate (1990)

Mineral oils, highly refined

Petroleum solvents (1989)

Printing inks (1996)

Tea (1991)

Terpene polychlorinates (StrobaneR) [8001-50-1]

Exposure circumstances

Flat-glass and specialty glass (manufacture of) (1993)

Hair colouring products (personal use of) (1993)

Leather goods manufacture

Leather tanning and processing

Lumber and sawmill industries (including logging)

Paint manufacture (occupational exposure in) (1989)

Pulp and paper manufacture

Group 4—Probably not carcinogenic to humans (1)

Caprolactam [105-60-2]

 

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Thursday, 10 March 2011 17:54

Occupational Exposure Limits

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The History of Occupational Exposure Limits

Over the past 40 years, many organizations in numerous countries have proposed occupational exposure limits (OELs) for airborne contaminants. The limits or guidelines that have gradually become the most widely accepted both in the United States and in most other countries are those issued annually by the American Conference of Governmental Industrial Hygienists (ACGIH), which are termed threshold limit values (TLVs) (LaNier 1984; Cook 1986; ACGIH 1994).

The usefulness of establishing OELs for potentially harmful agents in the working environment has been demonstrated repeatedly since their inception (Stokinger 1970; Cook 1986; Doull 1994). The contribution of OELs to the prevention or minimization of disease is now widely accepted, but for many years such limits did not exist, and even when they did, they were often not observed (Cook 1945; Smyth 1956; Stokinger 1981; LaNier 1984; Cook 1986).

It was well understood as long ago as the fifteenth century, that airborne dusts and chemicals could bring about illness and injury, but the concentrations and lengths of exposure at which this might be expected to occur were unclear (Ramazinni 1700).

As reported by Baetjer (1980), “early in this century when Dr. Alice Hamilton began her distinguished career in occupational disease, no air samples and no standards were available to her, nor indeed were they necessary. Simple observation of the working conditions and the illness and deaths of the workers readily proved that harmful exposures existed. Soon however, the need for determining standards for safe exposure became obvious.”

The earliest efforts to set an OEL were directed to carbon monoxide, the toxic gas to which more persons are occupationally exposed than to any other (for a chronology of the development of OELs, see figure 1. The work of Max Gruber at the Hygienic Institute at Munich was published in 1883. The paper described exposing two hens and twelve rabbits to known concentrations of carbon monoxide for up to 47 hours over three days; he stated that “the boundary of injurious action of carbon monoxide lies at a concentration in all probability of 500 parts per million, but certainly (not less than) 200 parts per million”. In arriving at this conclusion, Gruber had also inhaled carbon monoxide himself. He reported no symptoms or uncomfortable sensations after three hours on each of two consecutive days at concentrations of 210 parts per million and 240 parts per million (Cook 1986).

Figure 1. Chronology of occupational exposure levels (OELS).

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The earliest and most extensive series of animal experiments on exposure limits were those conducted by K.B. Lehmann and others under his direction. In a series of publications spanning 50 years they reported on studies on ammonia and hydrogen chloride gas, chlorinated hydrocarbons and a large number of other chemical substances (Lehmann 1886; Lehmann and Schmidt-Kehl 1936).

Kobert (1912) published one of the earlier tables of acute exposure limits. Concentrations for 20 substances were listed under the headings: (1) rapidly fatal to man and animals, (2) dangerous in 0.5 to one hour, (3) 0.5 to one hour without serious disturbances and (4) only minimal symptoms observed. In his paper “Interpretations of permissible limits”, Schrenk (1947) notes that the “values for hydrochloric acid, hydrogen cyanide, ammonia, chlorine and bromine as given under the heading ‘only minimal symptoms after several hours’ in the foregoing Kobert paper agree with values as usually accepted in present-day tables of MACs for reported exposures”. However, values for some of the more toxic organic solvents, such as benzene, carbon tetrachloride and carbon disulphide, far exceeded those currently in use (Cook 1986).

One of the first tables of exposure limits to originate in the United States was that published by the US Bureau of Mines (Fieldner, Katz and Kenney 1921). Although its title does not so indicate, the 33 substances listed are those encountered in workplaces. Cook (1986) also noted that most of the exposure limits through the 1930s, except for dusts, were based on rather short animal experiments. A notable exception was the study of chronic benzene exposure by Leonard Greenburg of the US Public Health Service, conducted under the direction of a committee of the National Safety Council (NSC 1926). An acceptable exposure for human beings based on long-term animal experiments was derived from this work.

According to Cook (1986), for dust exposures, permissible limits established before 1920 were based on exposures of workers in the South African gold mines, where the dust from drilling operations was high in crystalline free silica. In 1916, an exposure limit of 8.5 million particles per cubic foot of air (mppcf) for the dust with an 80 to 90% quartz content was set (Phthisis Prevention Committee 1916). Later, the level was lowered to 5 mppcf. Cook also reported that, in the United States, standards for dust, also based on exposure of workers, were recommended by Higgins and co-workers following a study at the south-western Missouri zinc and lead mines in 1917. The initial level established for high quartz dusts was ten mppcf, appreciably higher than was established by later dust studies conducted by the US Public Health Service. In 1930, the USSR Ministry of Labour issued a decree that included maximum allowable concentrations for 12 industrial toxic substances.

The most comprehensive list of occupational exposure limits up to 1926 was for 27 substances (Sayers 1927). In 1935 Sayers and Dalle Valle published physiological responses to five concentrations of 37 substances, the fifth being the maximum allowable concentration for prolonged exposure. Lehmann and Flury (1938) and Bowditch et al. (1940) published papers that presented tables with a single value for repeated exposures to each substance.

Many of the exposure limits developed by Lehmann were included in a monograph initially published in 1927 by Henderson and Haggard (1943), and a little later in Flury and Zernik’s Schadliche Gase (1931). According to Cook (1986), this book was considered the authoritative reference on effects of injurious gases, vapours and dusts in the workplace until Volume II of Patty’s Industrial Hygiene and Toxicology (1949) was published.

The first lists of standards for chemical exposures in industry, called maximum allowable concentrations (MACs), were prepared in 1939 and 1940 (Baetjer 1980). They represented a consensus of opinion of the American Standard Association and a number of industrial hygienists who had formed the ACGIH in 1938. These “suggested standards” were published in 1943 by James Sterner. A committee of the ACGIH met in early 1940 to begin the task of identifying safe levels of exposure to workplace chemicals, by assembling all the data which would relate the degree of exposure to a toxicant to the likelihood of producing an adverse effect (Stokinger 1981; LaNier 1984). The first set of values were released in 1941 by this committee, which was composed of Warren Cook, Manfred Boditch (reportedly the first hygienist employed by industry in the United States), William Fredrick, Philip Drinker, Lawrence Fairhall and Alan Dooley (Stokinger 1981).

In 1941, a committee (designated as Z-37) of the American Standards Association, which later became the American National Standards Institute, developed its first standard of 100 ppm for carbon monoxide. By 1974 the committee had issued separate bulletins for 33 exposure standards for toxic dusts and gases.

At the annual meeting of the ACGIH in 1942, the newly appointed Subcommittee on Threshold Limits presented in its report a table of 63 toxic substances with the “maximum allowable concentrations of atmospheric contaminants” from lists furnished by the various state industrial hygiene units. The report contains the statement, “The table is not to be construed as recommended safe concentrations. The material is presented without comment” (Cook 1986).

In 1945 a list of 132 industrial atmospheric contaminants with maximum allowable concentrations was published by Cook, including the then current values for six states, as well as values presented as a guide for occupational disease control by federal agencies and maximum allowable concentrations that appeared best supported by the references on original investigations (Cook 1986).

At the 1946 annual meeting of ACGIH, the Subcommittee on Threshold Limits presented their second report with the values of 131 gases, vapours, dusts, fumes and mists, and 13 mineral dusts. The values were compiled from the list reported by the subcommittee in 1942, from the list published by Warren Cook in Industrial Medicine (1945) and from published values of the Z-37 Committee of the American Standards Association. The committee emphasized that the “list of M.A.C. values is presented … with the definite understanding that it be subject to annual revision.”

Intended use of OELs

The ACGIH TLVs and most other OELs used in the United States and some other countries are limits which refer to airborne concentrations of substances and represent conditions under which “it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects” (ACGIH 1994). (See table 1).  In some countries the OEL is set at a concentration which will protect virtually everyone. It is important to recognize that unlike some exposure limits for ambient air pollutants, contaminated water, or food additives set by other professional groups or regulatory agencies, exposure to the TLV will not necessarily prevent discomfort or injury for everyone who is exposed (Adkins et al. 1990). The ACGIH recognized long ago that because of the wide range in individual susceptibility, a small percentage of workers may experience discomfort from some substances at concentrations at or below the threshold limit and that a smaller percentage may be affected more seriously by aggravation of a pre-existing condition or by development of an occupational illness (Cooper 1973; ACGIH 1994). This is clearly stated in the introduction to the ACGIH’s annual booklet Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices (ACGIH 1994).

Table 1. Occupational exposure limits (OELs) in various countries (as of 1986)

Country/Province

Type of standard

Argentina

OELs are essentially the same as those of the 1978 ACGIH TLVs. The principal difference from the ACGIH list is that, for the 144 substances (of the total of 630) for which no STELs are listed by ACGIH, the values used for the Argentina TWAs are entered also under this heading.

Australia

The National Health and Medical Research Council (NHMRC) adopted a revised edition of the Occupational Health Guide Threshold Limit Values (1990-91) in 1992. The OELs have no legal status in Australia, except where specifically incorporated into law by reference. The ACGIHTLVs are published in Australia as an appendix to the occupational health guides, revised with the ACGIH revisions in odd-numbered years.

Austria

The values recommended by the Expert Committee of the Worker Protection Commission for Appraisal of MAC (maximal acceptable concentration) Values in cooperation with the General Accident Prevention Institute of the Chemical Workers Trade Union, is considered obligatory by the Federal Ministry for Social Administration. They are applied by the Labour Inspectorate under the Labour Protection Law.

Belgium

The Administration of Hygiene and Occupational Medicine of the Ministry of Employment and of Labour uses the TLVs of the ACGIH as a guideline.

Brazil

The TLVs of the ACGIH have been used as the basis for the occupational health legislation of Brazil since 1978. As the Brazilian work week is usually 48 hours, the values of the ACGIH were adjusted in conformity with a formula developed for this purpose. The ACGIH list was adopted only for those air contaminants which at the time had nationwide application. The Ministry of Labour has brought the limits up to date with establishment of values for additional contaminants in accordance with recommendations from the Fundacentro Foundation of Occupational Safety and Medicine.

Canada (and Provinces)

Each province has its own regulations:

Alberta

OELs are under the Occupational Health and Safety Act, Chemical Hazard Regulation, which requires the employer to ensure that workers are not exposed above the limits.

British Columbia

The Industrial Health and Safety Regulations set legal requirements for most of British Columbia industry, which refer to the current schedule of TLVs for atmospheric contaminants published by the ACGIH.

Manitoba

The Department of Environment and Workplace Safety and Health is responsible for legislation and its administration concerning the OELs. The guidelines currently used to interpret risk to health are the ACGIH TLVs with the exception that carcinogens are given a zero exposure level “so far as is reasonably practicable”.

New Brunswick

The applicable standards are those published in the latest ACGIH issue and, in case of an infraction, it is the issue in publication at the time of infraction that dictates compliance.

Northwest Territories

The Northwest Territories Safety Division of the Justice and Service Department regulates workplace safety for non-federal employees under the latest edition of the ACGIH TLVs.

Nova Scotia

The list of OELs is the same as that of the ACGIH as published in 1976 and its subsequent amendments and revisions.

Ontario

Regulations for a number of hazardous substances are enforced under the Occupational Health and Safety Act, published each in a separate booklet that includes the permissible exposure level and codes for respiratory equipment, techniques for measuring airborne concentrations and medical surveillance approaches.

Quebec

Permissible exposure levels are similar to the ACGIH TLVs and compliance with the permissible exposure levels for workplace air contaminants is required.

Chile

The maximum concentration of eleven substances having the capacity of causing acute, severe or fatal effects cannot be exceeded for even a moment. The values in the Chile standard are those of the ACGIH TLVs to which a factor of 0.8 is applied in view of the 48-hour week.

Denmark

OELs include values for 542 chemical substances and 20 particulates. It is legally required that these not be exceeded as time-weighted averages. Data from the ACGIH are used in the preparation of the Danish standards. About 25 per cent of the values are different from those of ACGIH with nearly all of these being somewhat more stringent.

Ecuador

Ecuador does not have a list of permissible exposure levels incorporated in its legislation. The TLVs of the ACGIH are used as a guide for good industrial hygiene practice.

Finland

OELs are defined as concentrations that are deemed to be hazardous to at least some workers on long-term exposure. Whereas the ACGIH has as their philosophy that nearly all workers may be exposed to substances below the TLV without adverse effect, the viewpoint in Finland is that where exposures are above the limiting value, deleterious effects on health may occur.

Germany

The MAC value is “the maximum permissible concentration of a chemical compound present in the air within a working area (as gas, vapour, particulate matter) which, according to current knowledge, generally does not impair the health of the employee nor cause undue annoyance. Under these conditions, exposure can be repeated and of long duration over a daily period of eight hours, constituting an average work week of 40 hours (42 hours per week as averaged over four successive weeks for firms having four work shifts).- Scientifically based criteria for health protection, rather than their technical or economical feasibility, are employed.”

Ireland

The latest TLVs of the ACGIH are normally used. However, the ACGIH list is not incorporated in the national laws or regulations.

Netherlands

MAC values are taken largely from the list of the ACGIH, as well as from the Federal Republic of Germany and NIOSH. The MAC is defined as “that concentration in the workplace air which, according to present knowledge, after repeated long-term exposure even up to a whole working life, in general does not harm the health of workers or their offspring.”

Philippines

The 1970 TLVs of the ACGIH are used, except 50 ppm for vinyl chloride and 0.15 mg/m(3) for lead, inorganic compounds, fume and dust.

Russian Federation

The former USSR established many of its limits with the goal of eliminating any possibility for even reversible effects. Such subclinical and fully reversible responses to workplace exposures have, thus far, been considered too restrictive to be useful in the United States and in most other countries. In fact, due to the economic and engineering difficulties in achieving such low levels of air contaminants in the workplace, there is little indication that these limits have actually been achieved in countries which have adopted them. Instead, the limits appear to serve more as idealized goals rather than limits which manufacturers are legally bound or morally committed to achieve.

United States

At least six groups recommend exposure limits for the workplace: the TLVs of the ACGIH, the Recommended Exposure Limits (RELs) suggested by the National Institute for Occupational Safety and Health (NIOSH), the Workplace Environment Exposure Limits (WEEL) developed by the American Industrial Hygiene Association (AIHA), standards for workplace air contaminants suggested by the Z-37 Committee of the American National Standards Institute (EAL), the proposed workplace guides of the American Public Health Association (APHA 1991), and recommendations by local, state or regional governments. In addition, permissible exposure limits (PELs), which are regulations that must be met in the workplace because they are law, have been promulgated by the Department of Labor and are enforced by the Occupational Safety and Health Administration (OSHA).

Source: Cook 1986.

This limitation, although perhaps less than ideal, has been considered a practical one since airborne concentrations so low as to protect hypersusceptibles have traditionally been judged infeasible due to either engineering or economic limitations. Until about 1990, this shortcoming in the TLVs was not considered a serious one. In light of the dramatic improvements since the mid-1980s in our analytical capabilities, personal monitoring/sampling devices, biological monitoring techniques and the use of robots as a plausible engineering control, we are now technologically able to consider more stringent occupational exposure limits.

The background information and rationale for each TLV are published periodically in the Documentation of the Threshold Limit Values (ACGIH 1995). Some type of documentation is occasionally available for OELs set in other countries. The rationale or documentation for a particular OEL should always be consulted before interpreting or adjusting an exposure limit, as well as the specific data that were considered in establishing it (ACGIH 1994).

TLVs are based on the best available information from industrial experience and human and animal experimental studies—when possible, from a combination of these sources (Smith and Olishifski 1988; ACGIH 1994). The rationale for choosing limiting values differs from substance to substance. For example, protection against impairment of health may be a guiding factor for some, whereas reasonable freedom from irritation, narcosis, nuisance or other forms of stress may form the basis for others. The age and completeness of the information available for establishing occupational exposure limits also varies from substance to substance; consequently, the precision of each TLV is different. The most recent TLV and its documentation (or its equivalent) should always be consulted in order to evaluate the quality of the data upon which that value was set.

Even though all of the publications which contain OELs emphasize that they were intended for use only in establishing safe levels of exposure for persons in the workplace, they have been used at times in other situations. It is for this reason that all exposure limits should be interpreted and applied only by someone knowledgeable of industrial hygiene and toxicology. The TLV Committee (ACGIH 1994) did not intend that they be used, or modified for use:

  • as a relative index of hazard or toxicity
  • in the evaluation of community air pollution
  • for estimating the hazards of continuous, uninterrupted exposures or other extended work periods
  • as proof or disproof of an existing disease or physical condition
  • for adoption by countries whose working conditions differ from those of the United States.

 

The TLV Committee and other groups which set OELs warn that these values should not be “directly used” or extrapolated to predict safe levels of exposure for other exposure settings. However, if one understands the scientific rationale for the guideline and the appropriate approaches for extrapolating data, they can be used to predict acceptable levels of exposure for many different kinds of exposure scenarios and work schedules (ACGIH 1994; Hickey and Reist 1979).

Philosophy and approaches in setting exposure limits

TLVs were originally prepared to serve only for the use of industrial hygienists, who could exercise their own judgement in applying these values. They were not to be used for legal purposes (Baetjer 1980). However, in 1968 the United States Walsh-Healey Public Contract Act incorporated the 1968 TLV list, which covered about 400 chemicals. In the United States, when the Occupational Safety and Health Act (OSHA) was passed it required all standards to be national consensus standards or established federal standards.

Exposure limits for workplace air contaminants are based on the premise that, although all chemical substances are toxic at some concentration when experienced for a period of time, a concentration (e.g., dose) does exist for all substances at which no injurious effect should result no matter how often the exposure is repeated. A similar premise applies to substances whose effects are limited to irritation, narcosis, nuisance or other forms of stress (Stokinger 1981; ACGIH 1994).

This philosophy thus differs from that applied to physical agents such as ionizing radiation, and for some chemical carcinogens, since it is possible that there may be no threshold or no dose at which zero risk would be expected (Stokinger 1981). The issue of threshold effects is controversial, with reputable scientists arguing both for and against threshold theories (Seiler 1977; Watanabe et al. 1980, Stott et al. 1981; Butterworth and Slaga 1987; Bailer et al. 1988; Wilkinson 1988; Bus and Gibson 1994). With this in mind, some occupational exposure limits proposed by regulatory agencies in the early 1980s were set at levels which, although not completely without risk, posed risks that were no greater than classic occupational hazards such as electrocution, falls, and so on. Even in those settings which do not use industrial chemicals, the overall workplace risks of fatal injury are about one in one thousand. This is the rationale that has been used to justify selecting this theoretical cancer risk criterion for setting TLVs for chemical carcinogens (Rodricks, Brett and Wrenn 1987; Travis et al. 1987).

Occupational exposure limits established both in the United States and elsewhere are derived from a wide variety of sources. The 1968 TLVs (those adopted by OSHA in 1970 as federal regulations) were based largely on human experience. This may come as a surprise to many hygienists who have recently entered the profession, since it indicates that, in most cases, the setting of an exposure limit has come after a substance has been found to have toxic, irritational or otherwise undesirable effects on humans. As might be anticipated, many of the more recent exposure limits for systemic toxins, especially those internal limits set by manufacturers, have been based primarily on toxicology tests conducted on animals, in contrast to waiting for observations of adverse effects in exposed workers (Paustenbach and Langner 1986). However, even as far back as 1945, animal tests were acknowledged by the TLV Committee to be very valuable and they do, in fact, constitute the second most common source of information upon which these guidelines have been based (Stokinger 1970).

Several approaches for deriving OELs from animal data have been proposed and put into use over the past 40 years. The approach used by the TLV Committee and others is not markedly different from that which has been used by the US Food and Drug Administration (FDA) in establishing acceptable daily intakes (ADI) for food additives. An understanding of the FDA approach to setting exposure limits for food additives and contaminants can provide good insight to industrial hygienists who are involved in interpreting OELs (Dourson and Stara 1983).

Discussions of methodological approaches which can be used to establish workplace exposure limits based exclusively on animal data have also been presented (Weil 1972; WHO 1977; Zielhuis and van der Kreek 1979a, 1979b; Calabrese 1983; Dourson and Stara 1983; Leung and Paustenbach 1988a; Finley et al. 1992; Paustenbach 1995). Although these approaches have some degree of uncertainty, they seem to be much better than a qualitative extrapolation of animal test results to humans.

Approximately 50% of the 1968 TLVs were derived from human data, and approximately 30% were derived from animal data. By 1992, almost 50% were derived primarily from animal data. The criteria used to develop the TLVs may be classified into four groups: morphological, functional, biochemical and miscellaneous (nuisance, cosmetic). Of those TLVs based on human data, most are derived from effects observed in workers who were exposed to the substance for many years. Consequently, most of the existing TLVs have been based on the results of workplace monitoring, compiled with qualitative and quantitative observations of the human response (Stokinger 1970; Park and Snee 1983). In recent times, TLVs for new chemicals have been based primarily on the results of animal studies rather than human experience (Leung and Paustenbach 1988b; Leung et al. 1988).

It is noteworthy that in 1968 only about 50% of the TLVs were intended primarily to prevent systemic toxic effects. Roughly 40% were based on irritation and about two per cent were intended to prevent cancer. By 1993, about 50% were meant to prevent systemic effects, 35% to prevent irritation, and five per cent to prevent cancer. Figure 2 provides a summary of the data often used in developing OELs. 

Figure 2. Data often used in developing an occupational exposure.

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Limits for irritants

Prior to 1975, OELs designed to prevent irritation were largely based on human experiments. Since then, several experimental animal models have been developed (Kane and Alarie 1977; Alarie 1981; Abraham et al. 1990; Nielsen 1991). Another model based on chemical properties has been used to set preliminary OELs for organic acids and bases (Leung and Paustenbach 1988).

Limits for carcinogens

In 1972, the ACGIH Committee began to distinguish between human and animal carcinogens in its TLV list. According to Stokinger (1977), one reason for this distinction was to assist the stakeholders in discussions (union representatives, workers and the public) in focusing on those chemicals with more probable workplace exposures.

Do the TLVs Protect Enough Workers?

Beginning in 1988, concerns were raised by numerous persons regarding the adequacy or health protectiveness of TLVs. The key question raised was, what percentage of the working population is truly protected from adverse health effects when exposed to the TLV?

Castleman and Ziem (1988) and Ziem and Castleman (1989) argued both that the scientific basis of the standards was inadequate and that they were formulated by hygienists with vested interests in the industries being regulated.

These papers engendered an enormous amount of discussion, both supportive of and opposed to the work of the ACGIH (Finklea 1988; Paustenbach 1990a, 1990b, 1990c; Tarlau 1990).

A follow-up study by Roach and Rappaport (1990) attempted to quantify the safety margin and scientific validity of the TLVs. They concluded that there were serious inconsistencies between the scientific data available and the interpretation given in the 1976 Documentation by the TLV Committee. They also note that the TLVs were probably reflective of what the Committee perceived to be realistic and attainable at the time. Both the Roach and Rappaport and the Castleman and Ziem analyses have been responded to by the ACGIH, who have insisted on the inaccuracy of the criticisms.

Although the merit of the Roach and Rappaport analysis, or for that matter, that of Ziem and Castleman, will be debated for a number of years, it is clear that the process by which TLVs and other OELs will be set will probably never be as it was between 1945 and 1990. It is likely that in coming years, the rationale, as well as the degree of risk inherent in a TLV, will be more explicitly described in the documentation for each TLV. Also, it is certain that the definition of “virtually safe” or “insignificant risk” with respect to workplace exposure will change as the values of society change (Paustenbach 1995, 1997).

The degree of reduction in TLVs or other OELs that will undoubtedly occur in the coming years will vary depending on the type of adverse health effect to be prevented (central nervous system depression, acute toxicity, odour, irritation, developmental effects, or others). It is unclear to what degree the TLV committee will rely on various predictive toxicity models, or what risk criteria they will adopt, as we enter the next century.

Standards and Nontraditional Work Schedules

The degree to which shift work affects a worker’s capabilities, longevity, mortality, and overall well-being is still not well understood. So-called nontraditional work shifts and work schedules have been implemented in a number of industries in an attempt to eliminate, or at least reduce, some of the problems caused by normal shift work, which consists of three eight-hour work shifts per day. One kind of work schedule which is classified as nontraditional is the type involving work periods longer than eight hours and varying (compressing) the number of days worked per week (e.g., a 12-hours-per-day, three-day workweek). Another type of nontraditional work schedule involves a series of brief exposures to a chemical or physical agent during a given work schedule (e.g., a schedule where a person is exposed to a chemical for 30 minutes, five times per day with one hour between exposures). The last category of nontraditional schedule is that involving the “critical case” wherein persons are continuously exposed to an air contaminant (e.g., spacecraft, submarine).

Compressed workweeks are a type of nontraditional work schedule that has been used primarily in non-manufacturing settings. It refers to full-time employment (virtually 40 hours per week) which is accomplished in less than five days per week. Many compressed schedules are currently in use, but the most common are: (a) four-day workweeks with ten-hour days; (b) three-day workweeks with 12-hour days; (c) 4-1/2–day workweeks with four nine-hour days and one four-hour day (usually Friday); and (d) the five/four, nine plan of alternating five-day and four-day workweeks of nine-hour days (Nollen and Martin 1978; Nollen 1981).

Of all workers, those on nontraditional schedules represent only about 5% of the working population. Of this number, only about 50,000 to 200,000 Americans who work nontraditional schedules are employed in industries where there is routine exposure to significant levels of airborne chemicals. In Canada, the percentage of chemical workers on nontraditional schedules is thought to be greater (Paustenbach 1994).

One Approach to Setting International OELs

As noted by Lundberg (1994), a challenge facing all national committees is to identify a common scientific approach to setting OELs. Joint international ventures are advantageous to the parties involved since writing criteria documents is both a time- and cost-consuming process (Paustenbach 1995).

This was the idea when the Nordic Council of Ministers in 1977 decided to establish the Nordic Expert Group (NEG). The task of the NEG was to develop scientifically-based criteria documents to be used as a common scientific basis of OELs by the regulatory authorities in the five Nordic countries (Denmark, Finland, Iceland, Norway and Sweden). The criteria documents from the NEG lead to the definition of a critical effect and dose-response/dose-effect relationships. The critical effect is the adverse effect that occurs at the lowest exposure. There is no discussion of safety factors and a numerical OEL is not proposed. Since 1987, criteria documents are published by the NEG concurrently in English on a yearly basis.

Lundberg (1994) has suggested a standardized approach that each county would use. He suggested building a document with the following characteristics:

  • A standardized criteria document should reflect the up-to-date knowledge as presented in the scientific literature.
  • The literature used should preferably be peer-reviewed scientific papers but at least be available publicly. Personal communications should be avoided. An openness toward the general public, particularly workers, decreases the kind of suspiciousness that recently has been addressed toward documentation from the ACGIH.
  • The scientific committee should consist of independent scientists from academia and government. If the committee should include scientific representatives from the labour market, both employers and employees should be represented.
  • All relevant epidemiological and experimental studies should be thoroughly scrutinized by the scientific committee, especially “key studies” that present data on the critical effect. All observed effects should be described.
  • Environmental and biological monitoring possibilities should be pointed out. It is also necessary to thoroughly scrutinize these data, including toxicokinetic data.
  • Data permitting, the establishment of dose-response and dose-effect relationships should be stated. A no observable effect level (NOEL) or lowest observable effect level (LOEL) for each observed effect should be stated in the conclusion. If necessary, reasons should be given as to why a certain effect is the critical one. The toxicological significance of an effect is thereby considered.
  • Specifically, mutagenic, carcinogenic and teratogenic properties should be pointed out as well as allergic and immunological effects.
  • A reference list for all studies described should be given. If it is stated in the document that only relevant studies have been used, there is no need to give a list of references not used or why. On the other hand, it could be of interest to list those databases that have been used in the literature search.

 

There are in practice only minor differences in the way OELs are set in the various countries that develop them. It should, therefore, be relatively easy to agree upon the format of a standardized criteria document containing the key information. From this point, the decision as to the size of the margin of safety that is incorporated in the limit would then be a matter of national policy.

 

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Sunday, 16 January 2011 19:52

Carcinogen Risk Assessment

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Whereas the principles and methods of risk assessment for non-carcinogenic chemicals are similar in different parts of the world, it is striking that approaches for risk assessment of carcinogenic chemicals vary greatly. There are not only marked differences between countries, but even within a country different approaches are applied or advocated by various regulatory agencies, committees and scientists in the field of risk assessment. Risk assessment for non-carcinogens is rather consistent and pretty well established partly because of the long history and better understanding of the nature of toxic effects in comparison with carcinogens and a high degree of consensus and confidence by both scientists and the general public on methods used and their outcome.

For non-carcinogenic chemicals, safety factors were introduced to compensate for uncertainties in the toxicology data (which are derived mostly from animal experiments) and in their applicability to large, heterogeneous human populations. In doing so, recommended or required limits on safe human exposures were usually set at a fraction (the safety or uncertainty factor approach) of the exposure levels in animals that could be clearly documented as the no observed adverse effects level (NOAEL) or the lowest observed adverse effects level (LOAEL). It was then assumed that as long as human exposure did not exceed the recommended limits, the hazardous properties of chemical substances would not be manifest. For many types of chemicals, this practice, in somewhat refined form, continues to this day in toxicological risk assessment.

During the late 1960s and early 1970s regulatory bodies, starting in the United States, were confronted with an increasingly important problem for which many scientists considered the safety factor approach to be inappropriate, and even dangerous. This was the problem with chemicals that under certain conditions had been shown to increase the risk of cancers in humans or experimental animals. These substances were operationally referred to as carcinogens. There is still debate and controversy on the definition of a carcinogen, and there is a wide range of opinion about techniques to identify and classify carcinogens and the process of cancer induction by chemicals as well.

The initial discussion started much earlier, when scientists in the 1940s discovered that chemical carcinogens caused damage by a biological mechanism that was of a totally different kind from those that produced other forms of toxicity. These scientists, using principles from the biology of radiation-induced cancers, put forth what is referred to as the “non-threshold” hypothesis, which was considered applicable to both radiation and carcinogenic chemicals. It was hypothesized that any exposure to a carcinogen that reaches its critical biological target, especially the genetic material, and interacts with it, can increase the probability (the risk) of cancer development.

Parallel to the ongoing scientific discussion on thresholds, there was a growing public concern on the adverse role of chemical carcinogens and the urgent need to protect the people from a set of diseases collectively called cancer. Cancer, with its insidious character and long latency period together with data showing that cancer incidences in the general population were increasing, was regarded by the general public and politicians as a matter of concern that warranted optimal protection. Regulators were faced with the problem of situations in which large numbers of people, sometimes nearly the entire population, were or could be exposed to relatively low levels of chemical substances (in consumer products and medicines, at the workplace as well as in air, water, food and soils) that had been identified as carcinogenic in humans or experimental animals under conditions of relatively intense exposures.

Those regulatory officials were confronted with two fundamental questions which, in most cases, could not be fully answered using available scientific methods:

  1.  What risk to human health exists in the range of exposure to chemicals below the relatively intense and narrow exposure range under which a cancer risk could be directly measured?
  2.  What could be said about risks to human health when experimental animals were the only subjects in which risks for the development of cancer had been established?

 

Regulators recognized the need for assumptions, sometimes scientifically based but often also unsupported by experimental evidence. In order to achieve consistency, definitions and specific sets of assumptions were adapted that would be generically applied to all carcinogens.

Carcinogenesis Is a Multistage Process

Several lines of evidence support the conclusion that chemical carcinogenesis is a multistage process driven by genetic damage and epigenetic changes, and this theory is widely accepted in the scientific community all over the world (Barrett 1993). Although the process of chemical carcinogenesis is often separated into three stages—initiation, promotion and progression—the number of relevant genetic changes is not known.

Initiation involves the induction of an irreversibly altered cell and is for genotoxic carcinogens always equated with a mutational event. Mutagenesis as a mechanism of carcinogenesis was already hypothesized by Theodor Boveri in 1914, and many of his assumptions and predictions have subsequently been proven to be true. Because irreversible and self-replicating mutagenic effects can be caused by the smallest amount of a DNA-modifying carcinogen, no threshold is assumed. Promotion is the process by which the initiated cell expands (clonally) by a series of divisions, and forms (pre)neoplastic lesions. There is considerable debate as to whether during this promotion phase initiated cells undergo additional genetic changes.

Finally in the progression stage “immortality” is obtained and full malignant tumours can develop by influencing angiogenesis, escaping the reaction of the host control systems. It is characterized by invasive growth and frequently metastatic spread of the tumour. Progression is accompanied by additional genetic changes due to the instability of proliferating cells and selection.

Therefore, there are three general mechanisms by which a substance can influence the multistep carcinogenic process. A chemical can induce a relevant genetic alteration, promote or facilitate clonal expansion of an initiated cell or stimulate progression to malignancy by somatic and/or genetic changes.

Risk Assessment Process

Risk can be defined as the predicted or actual frequency of occurrence of an adverse effect on humans or the environment, from a given exposure to a hazard. Risk assessment is a method of systematically organizing the scientific information and its attached uncertainties for description and qualification of the health risks associated with hazardous substances, processes, actions or events. It requires evaluation of relevant information and selection of the models to be used in drawing inferences from that information. Further, it requires explicit recognition of uncertainties and appropriate acknowledgement that alternative interpretation of the available data may be scientifically plausible. The current terminology used in risk assessment was proposed in 1984 by the US National Academy of Sciences. Qualitative risk assessment changed into hazard characterization/identification and quantitative risk assessment was divided into the components dose-response, exposure assessment and risk characterization.

In the following section these components will be briefly discussed in view of our current knowledge of the process of (chemical) carcinogenesis. It will become clear that the dominant uncertainty in the risk assessment of carcinogens is the dose-response pattern at low dose levels characteristic for environmental exposure.

Hazard identification

This process identifies which compounds have the potential to cause cancer in humans—in other words it identifies their intrinsic genotoxic properties. Combining information from various sources and on different properties serves as a basis for classification of carcinogenic compounds. In general the following information will be used:

  • epidemiological data (e.g., vinylchloride, arsenic, asbestos)
  • animal carcinogenicity data
  • genotoxic activity/DNA adduct formation
  • mechanisms of action
  • pharmacokinetic activity
  • structure-activity relationships.

 

Classification of chemicals into groups based on the assessment of the adequacy of the evidence of carcinogenesis in animals or in man, if epidemiological data are available, is a key process in hazard identification. The best known schemes for categorizing carcinogenic chemicals are those of IARC (1987), EU (1991) and the EPA (1986). An overview of their criteria for classification (e.g., low-dose extrapolation methods) is given in table 1.

Table 1. Comparison of low-dose extrapolations procedures

  Current US EPA Denmark EEC UK Netherlands Norway
Genotoxic carcinogen Linearized multistage procedure using most appropriate low-dose model MLE from 1- and 2-hit models plus judgement of best outcome No procedure specified No model, scientific expertise and judgement from all available data Linear model using TD50 (Peto method) or “Simple Dutch Method” if no TD50 No procedure specified
Non-genotoxic carcinogen Same as above Biologically-based model of Thorslund or multistage or Mantel-Bryan model, based on tumour origin and dose-response Use NOAEL and safety factors Use NOEL and safety factors to set ADI Use NOEL and safety factors to set ADI  

 

One important issue in classifying carcinogens, with sometimes far-reaching consequences for their regulation, is the distinction between genotoxic and non-genotoxic mechanisms of action. The US Environmental Protection Agency (EPA) default assumption for all substances showing carcinogenic activity in animal experiments is that no threshold exists (or at least none can be demonstrated), so there is some risk with any exposure. This is com- monly referred to as the non-threshold assumption for genotoxic (DNA-damaging) compounds. The EU and many of its members, such as the United Kingdom, the Netherlands and Denmark, make a distinction between carcinogens that are genotoxic and those believed to produce tumours by non-genotoxic mechanisms. For genotoxic carcinogens quantitative dose-response estimation procedures are followed that assume no threshold, although the procedures might differ from those used by the EPA. For non-genotoxic substances it is assumed that a threshold exists, and dose-response procedures are used that assume a threshold. In the latter case, the risk assessment is generally based on a safety factor approach, similar to the approach for non-carcinogens.

It is important to keep in mind that these different schemes were developed to deal with risk assessments in different contexts and settings. The IARC scheme was not produced for regulatory purposes, although it has been used as a basis for developing regulatory guidelines. The EPA scheme was designed to serve as a decision point for entering quantitative risk assessment, whereas the EU scheme is currently used to assign a hazard (classification) symbol and risk phrases to the chemical's label. A more extended discussion on this subject is presented in a recent review (Moolenaar 1994) covering procedures used by eight governmental agencies and two often-cited independent organizations, the Inter- national Agency for Research on Cancer (IARC) and the American Conference of Governmental Industrial Hygienists (ACGIH).

The classification schemes generally do not take into account the extensive negative evidence that may be available. Also, in recent years a greater understanding of the mechanism of action of carcinogens has emerged. Evidence has accumulated that some mechanisms of carcinogenicity are species-specific and are not relevant for man. The following examples will illustrate this important phenomenon. First, it has been recently demonstrated in studies on the carcinogenicity of diesel particles, that rats respond with lung tumours to a heavy loading of the lung with particles. However, lung cancer is not seen in coal miners with very heavy lung burdens of particles. Secondly, there is the assertion of the nonrelevance of renal tumours in the male rat on the basis that the key element in the tumourgenic response is the accumulation in the kidney of α-2 microglobulin, a protein that does not exist in humans (Borghoff, Short and Swenberg 1990). Disturbances of rodent thyroid function and peroxisome proliferation or mitogenesis in the mouse liver have also to be mentioned in this respect.

This knowledge allows a more sophisticated interpretation of the results of a carcinogenicity bioassay. Research towards a better understanding of the mechanisms of action of carcinogenicity is encouraged because it may lead to an altered classification and to the addition of a category in which chemicals are classified as not carcinogenic to humans.

Exposure assessment

Exposure assessment is often thought to be the component of risk assessment with the least inherent uncertainty because of the ability to monitor exposures in some cases and the availability of relatively well-validated exposure models. This is only partially true, however, because most exposure assessments are not conducted in ways that take full advantage of the range of available information. For that reason there is a great deal of room for improving exposure distribution estimates. This holds for both external as well as for internal exposure assessments. Especially for carcinogens, the use of target tissue doses rather than external exposure levels in modelling dose-response relationships would lead to more relevant predictions of risk, although many assumptions on default values are involved. Physiologically based pharmacokinetic (PBPK) models to determine the amount of reactive metabolites that reaches the target tissue are potentially of great value to estimate these tissue doses.

Risk Characterization

Current approaches

The dose level or exposure level that causes an effect in an animal study and the likely dose causing a similar effect in humans is a key consideration in risk characterization. This includes both dose-response assessment from high to low dose and interspecies extrapolation. The extrapolation presents a logical problem, namely that data are being extrapolated many orders of magnitude below the experimental exposure levels by empirical models that do not reflect the underlying mechanisms for carcinogenicity. This violates a basic principle in fitting of empirical models, namely not to extrapolate outside the range of the observable data. Therefore, this empirical extrapolation results in large uncertainties, both from a statistical and from a biological point of view. At present no single mathematical procedure is recognized as the most appropriate one for low-dose extrapolation in carcinogenesis. The mathematical models that have been used to describe the relation between the administered external dose, the time and the tumour incidence are based on either tolerance-distribution or mechanistic assumptions, and sometimes based on both. A summary of the most frequently cited models (Kramer et al. 1995) is listed in table 2.

Table 2. Frequently cited models in carcinogen risk characterization

Tolerance distribution models Mechanistic models  
  Hit-models Biologically based models
Logit One-hit Moolgavkar (MVK)1
Probit Multihit Cohen and Ellwein
Mantel-Bryan Weibull (Pike)1  
Weibull Multistage (Armitage-Doll)1  
Gamma Multihit Linearized Multistage,  

1 Time-to-tumour models.

These dose-response models are usually applied to tumour-incidence data corresponding to only a limited number of experimental doses. This is due to the standard design of the applied bioassay. Instead of determining the complete dose-response curve, a carcinogenicity study is in general limited to three (or two) relatively high doses, using the maximum tolerated dose (MTD) as highest dose. These high doses are used to overcome the inherent low statistical sensitivity (10 to 15% over background) of such bioassays, which is due to the fact that (for practical and other reasons) a relatively small number of animals is used. Because data for the low-dose region are not available (i.e., cannot be determined experimentally), extrapolation outside the range of observation is required. For almost all data sets, most of the above-listed models fit equally well in the observed dose range, due to the limited number of doses and animals. However, in the low-dose region these models diverge several orders of magnitude, thereby introducing large uncertainties to the risk estimated for these low exposure levels.

Because the actual form of the dose-response curve in the low-dose range cannot be generated experimentally, mechanistic insight into the process of carcinogenicity is crucial to be able to discriminate on this aspect between the various models. Comprehensive reviews discussing the various aspects of the different mathematical extrapolation models are presented in Kramer et al. (1995) and Park and Hawkins (1993).

Other approaches

Besides the current practice of mathematical modelling several alternative approaches have been proposed recently.

Biologically motivated models

Currently, the biologically based models such as the Moolgavkar-Venzon-Knudson (MVK) models are very promising, but at present these are not sufficiently well advanced for routine use and require much more specific information than currently is obtained in bioassays. Large studies (4,000 rats) such as those carried out on N-nitrosoalkylamines indicate the size of the study which is required for the collection of such data, although it is still not possible to extrapolate to low doses. Until these models are further developed they can be used only on a case-by-case basis.

Assessment factor approach

The use of mathematical models for extrapolation below the experimental dose range is in effect equivalent to a safety factor approach with a large and ill-defined uncertainty factor. The simplest alternative would be to apply an assessment factor to the apparent “no effect level”, or the “lowest level tested”. The level used for this assessment factor should be determined on a case-by-case basis considering the nature of the chemical and the population being exposed.

Benchmark dose (BMD)

The basis of this approach is a mathematical model fitted to the experimental data within the observable range to estimate or interpolate a dose corresponding to a defined level of effect, such as one, five or ten per cent increase in tumour incidence (ED01, ED05, ED10). As a ten per cent increase is about the smallest change that statistically can be determined in a standard bioassay, the ED10 is appropriate for cancer data. Using a BMD that is within the observable range of the experiment avoids the problems associated with dose extrapolation. Estimates of the BMD or its lower confidence limit reflect the doses at which changes in tumour incidence occurred, but are quite insensitive to the mathematical model used. A benchmark dose can be used in risk assessment as a measure of tumour potency and combined with appropriate assessment factors to set acceptable levels for human exposure.

Threshold of regulation

Krewski et al. (1990) have reviewed the concept of a “threshold of regulation” for chemical carcinogens. Based on data obtained from the carcinogen potency database (CPDB) for 585 experiments, the dose corresponding to 10-6 risk was roughly log-normally distributed around a median of 70 to 90 ng/kg/d. Exposure to dose levels greater than this range would be considered unacceptable. The dose was estimated by linear extrapolation from the TD50 (the dose inducing toxicity is 50% of the animals tested) and was within a factor of five to ten of the figure obtained from the linearized multistage model. Unfortunately, the TD50 values will be related to the MTD, which again casts doubt on the validity of the measurement. However the TD50 will often be within or very close to the experimental data range.

Such an approach as using a threshold of regulation would require much more consideration of biological, analytical and mathematical issues and a much wider database before it could be considered. Further investigation into the potencies of various carcinogens may throw further light onto this area.

Objectives and Future of CarcinogenRisk Assessment

Looking back to the original expectations on the regulation of (environmental) carcinogens, namely to achieve a major reduction in cancer, it appears that the results at present are disappointing. Over the years it became apparent that the number of cancer cases estimated to be produced by regulatable carcinogens was disconcertingly small. Considering the high expectations that launched the regulatory efforts in the 1970s, a major anticipated reduction in the cancer death rate has not been achieved in terms of the estimated effects of environmental carcinogens, not even with ultraconservative quantitative assessment procedures. The main characteristic of the EPA procedures is that low-dose extrapolations are made in the same way for each chemical regardless of the mechanism of tumour formation in experimental studies. It should be noted, however, that this approach stands in sharp contrast to approaches taken by other governmental agencies. As indicated above, the EU and several European governments—Denmark, France, Germany, Italy, the Netherlands, Sweden, Switzerland, UK—distinguish between genotoxic and non-genotoxic carcinogens, and approach risk estimation differently for the two categories. In general, non-genotoxic carcinogens are treated as threshold toxicants. No effect levels are determined, and uncertainty factors are used to provide an ample margin of safety. To determine whether or not a chemical should be regarded as non-genotoxic is a matter of scientific debate and requires clear expert judgement.

The fundamental issue is: What is the cause of cancer in humans and what is the role of environmental carcinogens in that causation? The hereditary aspects of cancer in humans are much more important than previously anticipated. The key to signifi- cant advancement in the risk assessment of carcinogens is a better understanding of the causes and mechanisms of cancer. The field of cancer research is entering a very exciting area. Molecular research may radically alter the way we view the impact of environmental carcinogens and the approaches to control and prevent cancer, both for the general public and the workplace. Risk assessment of carcinogens needs to be based on concepts of the mechanisms of action that are, in fact, just emerging. One of the important aspects is the mechanism of heritable cancer and the interaction of carcinogens with this process. This knowledge will have to be incorporated into the systematic and consistent methodology that already exists for the risk assessment of carcinogens.

 

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Monday, 14 March 2011 19:45

Workstations

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An Integrated Approach in the Design of Workstations

In ergonomics, the design of workstations is a critical task. There is general agreement that in any work setting, whether blue-collar or white-collar, a well-designed workstation furthers not only the health and well-being of the workers, but also productivity and the quality of the products. Conversely, the poorly designed workstation is likely to cause or contribute to the development of health complaints or chronic occupational diseases, as well as to problems with keeping product quality and productivity at a prescribed level.

To every ergonomist, the above statement may seem trivial. It is also recognized by every ergonomist that working life worldwide is full of not only ergonomic shortcomings, but blatant violations of basic ergonomic principles. It is clearly evident that there is a widespread unawareness with respect to the importance of workstation design among those responsible: production engineers, supervisors and managers.

It is noteworthy that there is an international trend with respect to industrial work which would seem to underline the importance of ergonomic factors: the increasing demand for improved product quality, flexibility and product delivery precision. These demands are not compatible with a conservative view regarding the design of work and workplaces.

Although in the present context it is the physical factors of workplace design that are of chief concern, it should be borne in mind that the physical design of the workstation cannot in practice be separated from the organization of work. This principle will be made evident in the design process described in what follows. The quality of the end result of the process relies on three supports: ergonomic knowledge, integration with productivity and quality demands, and participation. The process of implementation of a new workstation must cater to this integration, and it is the main focus of this article.

Design considerations

Workstations are meant for work. It must be recognized that the point of departure in the workstation design process is that a certain production goal has to be achieved. The designer—often a production engineer or other person at middle-management level—develops internally a vision of the workplace, and starts to implement that vision through his or her planning media. The process is iterative: from a crude first attempt, the solutions become gradually more and more refined. It is essential that ergonomic aspects be taken into account in each iteration as the work progresses.

It should be noted that ergonomic design of workstations is closely related to ergonomic assessment of workstations. In fact, the structure to be followed here applies equally to the cases where the workstation already exists or when it is in a planning stage.

In the design process, there is a need for a structure which ensures that all relevant aspects be considered. The traditional way to handle this is to use checklists containing a series of those variables which should be taken into account. However, general purpose checklists tend to be voluminous and difficult to use, since in a particular design situation only a fraction of the checklist may be relevant. Furthermore, in a practical design situation, some variables stand out as being more important than others. A methodology to consider these factors jointly in a design situation is required. Such a methodology will be proposed in this article.

Recommendations for workstation design must be based on a relevant set of demands. It should be noted that it is in general not enough to take into account threshold limit values for individual variables. A recognized combined goal of productivity and conservation of health makes it necessary to be more ambitious than in a traditional design situation. In particular, the question of musculoskeletal complaints is a major aspect in many industrial situations, although this category of problems is by no means limited to the industrial environment.

A Workstation Design Process

Steps in the process

In the workstation design and implementation process, there is always an initial need to inform users and to organize the project so as to allow for full user participation and in order to increase the chance of full employee acceptance of the final result. A treatment of this goal is not within the scope of the present treatise, which concentrates on the problem of arriving at an optimal solution for the physical design of the workstation, but the design process nonetheless allows the integration of such a goal. In this process, the following steps should always be considered:

    1. collection of user-specified demands
    2. prioritizing of demands
    3. transfer of demands into (a) technical specifications and (b) specifications in user terms
    4. iterative development of the workstation’s physical layout
    5. physical implementation
    6. trial period of production
    7. full production
    8. evaluation and identification of rest problems.

                   

                  The focus here is on steps one through five. Many times, only a subset of all these steps is actually included in the design of workstations. There may be various reasons for this. If the workstation is a standard design, such as in some VDU working situations, some steps may duly be excluded. However, in most cases the exclusion of some of the steps listed would lead to a workstation of lower quality than what can be considered acceptable. This can be the case when economic or time constraints are too severe, or when there is sheer neglect due to lack of knowledge or insight at management level.

                  Collection of user-specified demands

                  It is essential to identify the user of the workplace as any member of the production organization who may be able to contribute qualified views on its design. Users may include, for instance, the workers, the supervisors, the production planners and production engineers, as well as the safety steward. Experience shows clearly that these actors all have their unique knowledge which should be made use of in the process.

                  The collection of the user-specified demands should meet a number of criteria:

                  1. Openness. There should be no filter applied in the initial stage of the process. All points of view should be noted without voiced criticism.
                  2. Non-discrimination. Viewpoints from every category should be treated equally at this stage of the process. Special consideration should be given to the fact that some persons may be more outspoken than others, and that there is a risk that they may silence some of the other actors.
                  3. Development through dialogue. There should be an opportunity to adjust and develop the demands through a dialogue between participants of different backgrounds. Prioritizing should be addressed as part of the process.
                  4. Versatility. The process of collection of user-specified demands should be reasonably economical and not require the involvement of specialist consultants or extensive time demands on the part of the participants.

                   

                  The above set of criteria may be met by using a methodology based on quality function deployment (QFD) according to Sullivan (1986). Here, the user demands may be collected in a session where a mixed group of actors (not more than eight to ten people) is present. All participants are given a pad of removable self-sticking notes. They are asked to write down all workplace demands which they find relevant, each one on a separate slip of paper. Aspects relating to work environment and safety, productivity and quality should be covered. This activity may continue for as long as found necessary, typically ten to fifteen minutes. After this session, one after the other of the participants is asked to read out his or her demands and to stick the notes on a board in the room where everyone in the group can see them. The demands are grouped into natural categories such as lighting, lifting aids, production equipment, reaching requirements and flexibility demands. After the completion of the round, the group is given the opportunity to discuss and to comment on the set of demands, one category at a time, with respect to relevance and priority.

                  The set of user-specified demands collected in a process such as the one described in the above forms one of the bases for the development of the demand specification. Additional information in the process may be produced by other categories of actors, for example, product designers, quality engineers, or economists; however, it is vital to realize the potential contribution that the users can make in this context.

                  Prioritizing and demand specification

                  With respect to the specification process, it is essential that the different types of demands be given consideration according to their respective importance; otherwise, all aspects that have been taken into account will have to be considered in parallel, which may tend to make the design situation complex and difficult to handle. This is why checklists, which need to be elaborate if they are to serve the purpose, tend to be difficult to manage in a particular design situation.

                  It may be difficult to devise a priority scheme which serves all types of workstations equally well. However, on the assumption that manual handling of materials, tools or products is an essential aspect of the work to be carried out in the workstation, there is a high probability that aspects associated with musculoskeletal load will be at the top of the priority list. The validity of this assumption may be checked in the user demand collection stage of the process. Relevant user demands may be, for instance, associated with muscular strain and fatigue, reaching, seeing, or ease of manipulation.

                  It is essential to realize that it may not be possible to transform all user-specified demands into technical demand specifications. Although such demands may relate to more subtle aspects such as comfort, they may nevertheless be of high relevance and should be considered in the process.

                  Musculoskeletal load variables

                  In line with the above reasoning, we shall here apply the view that there is a set of basic ergonomic variables relating to musculoskeletal load which need to be taken into account as a priority in the design process, in order to eliminate the risk of work-related musculosketal disorders (WRMDs). This type of disorder is a pain syndrome, localized in the musculoskeletal system, which develops over long periods of time as a result of repeated stresses on a particular body part (Putz-Anderson 1988). The essential variables are (e.g., Corlett 1988):

                  • muscular force demand
                  • working posture demand
                  • time demand.

                   

                  With respect to muscular force, criteria setting may be based on a combination of biomechanical, physiological and psychological factors. This is a variable that is operationalized through measurement of output force demands, in terms of handled mass or required force for, say, the operation of handles. Also, peak loads in connection with highly dynamic work may have to be taken into account.

                  Working posture demands may be evaluated by mapping (a) situations where the joint structures are stretched beyond the natural range of movement, and (b) certain particularly awkward situations, such as kneeling, twisting, or stooped postures, or work with the hand held above shoulder level.

                  Time demands may be evaluated on the basis of mapping (a) short-cycle, repetitive work, and (b) static work. It should be noted that static work evaluation may not exclusively concern maintaining a working posture or producing a constant output force over lengthy periods of time; from the point of view of the stabilizing muscles, particularly in the shoulder joint, seemingly dynamic work may have a static character. It may thus be necessary to consider lengthy periods of joint mobilization.

                  The acceptability of a situation is of course based in practice on the demands on the part of the body that is under the highest strain.

                  It is important to note that these variables should not be considered one at a time but jointly. For instance, high force demands may be acceptable if they occur only occasionally; lifting the arm above shoulder level once in a while is not normally a risk factor. But combinations among such basic variables must be considered. This tends to make criteria setting difficult and involved.

                  In the Revised NIOSH equation for the design and evaluation of manual handling tasks (Waters et al. 1993), this problem is addressed by devising an equation for recommended weight limits which takes into account the following mediating factors: horizontal distance, vertical lifting height, lifting asymmetry, handle coupling and lifting frequency. In this way, the 23-kilogram acceptable load limit based on biomechanical, physiological and psychological criteria under ideal conditions, may be modified substantially upon taking into account the specifics of the working situation. The NIOSH equation provides a base for evaluation of work and workplaces involving lifting tasks. However, there are severe limitations as to the usability of the NIOSH equation: for instance, only two-handed lifts may be analysed; scientific evidence for analysis of one-handed lifts is still inconclusive. This illustrates the problem of applying scientific evidence exclusively as a basis for work and workplace design: in practice, scientific evidence must be merged with educated views of persons who have direct or indirect experience of the type of work considered.

                  The cube model

                  Ergonomic evaluation of workplaces, taking into account the complex set of variables which need to be considered, is to a large extent a communications problem. Based on the prioritizing discussion described above, a cube model for ergonomic evaluation of workplaces was developed (Kadefors 1993). Here the prime goal was to develop a didactic tool for communication purposes, based on the assumption that output force, posture and time measures in a great majority of situations constitute interrelated, prioritized basic variables.

                  For each one of the basic variables, it is recognized that the demands may be grouped with respect to severity. Here, it is proposed that such a grouping may be made in three classes: (1) low demands, (2) medium demands or (3) high demands. The demand levels may be set either by using whatever scientific evidence is available or by taking a consensus approach with a panel of users. These two alternatives are of course not mutually exclusive, and may well entail similar results, but probably with different degrees of generality.

                  As noted above, combinations of the basic variables determine to a large extent the risk level with respect to the development of musculoskeletal complaints and cumulative trauma disorders. For instance, high time demands may render a working situation unacceptable in cases where there are also at least medium level demands with respect to force and posture. It is essential in the design and assessment of workplaces that the most important variables be considered jointly. Here a cube model for such evaluation purposes is proposed. The basic variables—force, posture and time—constitute the three axes of the cube. For each combination of demands a subcube may be defined; in all, the model incorporates 27 such subcubes (see figure 1).

                  Figure 1. The "cube model" for ergonomics assessment. Each cube represents a combination of demands relating to force, posture and time. Light: acceptable combination; gray: conditionally acceptable; black: unacceptable

                  ERG190F1

                  An essential aspect of the model is the degree of acceptability of the demand combinations. In the model, a three-zone classification scheme is proposed for acceptability: (1) the situation is acceptable, (2) the situation is conditionally acceptable or (3) the situation is unacceptable. For didactic purposes, each subcube may be given a certain texture or colour (say, green-yellow-red). Again, the assessment may be user-based or based on scientific evidence. The conditionally acceptable (yellow) zone means that “there exists a risk of disease or injury that cannot be neglected, for the whole or a part of the operator population in question” (CEN 1994).

                  In order to develop this approach, it is useful to consider a case: the evaluation of load on the shoulder in moderately paced one-handed materials handling. This is a good example, since in this type of situation, it is normally the shoulder structures that are under the heaviest strain.

                  With respect to the force variable, classification may be based in this case on handled mass. Here, low force demand is identified as levels below 10% of maximal voluntary lifting capacity (MVLC), which amounts to approximately 1.6 kg in an optimal working zone. High force demand requires more than 30% MVLC, approximately 4.8 kg. Medium force demand falls in between these limits. Low postural strain is when the upper arm is close to the thorax. High postural strain is when humeral abduction or flexion exceeds 45°. Medium postural strain is when the abduction/flexion angle is between 15° and 45°. Low time demand is when the handling occupies less than one hour per working day on and off, or continuously for less than 10 minutes per day. High time demand is when the handling takes place for more than four hours per working day, or continuously for more than 30 minutes (sustained or repetitively). Medium time demand is when the exposure falls between these limits.

                  In figure 1, degrees of acceptability have been assigned to combinations of demands. For instance, it is seen that high time demands may only be combined with combined low force and postural demands. Moving from unacceptable to acceptable may be undertaken by reducing demands in either dimension, but reduction in time demands is the most efficient way in many cases. In other words, in some cases workplace design should be altered, in other cases it may be more efficient to change the organization of work.

                  Using a consensus panel with a set of users for definition of demand levels and classification of degree of acceptability may enhance the workstation design process considerably, as considered below.

                  Additional variables

                  In addition to the basic variables considered above, a set of variables and factors characterizing the workplace from an ergonomics point of view has to be taken into account, depending upon the particular conditions of the situation to be analysed. They include:

                  • precautions to reduce risks for accidents
                  • specific environmental factors such as noise, lighting and ventilation
                  • exposure to climatic factors
                  • exposure to vibration (from hand-held tools or whole body)
                  • ease of meeting productivity and quality demands.

                   

                  To a large extent these factors may be considered one at a time; hence the checklist approach may be useful. Grandjean (1988) in his textbook covers the essential aspects that usually need to be taken into account in this context. Konz (1990) in his guidelines provides for workstation organization and design a set of leading questions focusing on worker-machine interfacing in manufacturing systems.

                  In the design process followed here, the checklist should be read in conjunction with the user-specified demands.

                  A Workstation Design Example: Manual Welding

                  As an illustrative (hypothetical) example, the design process leading to implementation of a workstation for manual welding (Sundin et al. 1994) is described here. Welding is an activity frequently combining high demands for muscular force with high demands for manual precision. The work has a static character. The welder is often doing welding exclusively. The welding work environment is generally hostile, with a combination of exposure to high noise levels, welding smoke and optical radiation.

                  The task was to devise a workplace for manual MIG (metal inert gas) welding of medium size objects (up to 300 kg) in a workshop environment. The workstation had to be flexible since there was a variety of objects to be manufactured. There were high demands for productivity and quality.

                  A QFD process was carried out in order to provide a set of workstation demands in user terms. Welders, production engineers and product designers were involved. User demands, which are not listed here, covered a wide range of aspects including ergonomics, safety, productivity and quality.

                  Using the cube model approach, the panel identified, by consensus, limits between high, moderate and low load:

                    1. Force variable. Less than 1 kg handled mass is termed a low load, whereas more than 3 kg is considered a high load.
                    2. Postural strain variable. Working positions implying high strain are those involving elevated arms, twisted or deep forward-flexed positions, and kneeling positions, and also include situations where the wrist is held in extreme flexion/extension or deviation. Low strain occurs where the posture is straight upright standing or sitting and where hands are in optimal working zones.
                    3. Time variable. Less than 10% of the working time devoted to welding is considered low demand, whereas more than 40% of total working time is termed high demand. Medium demands occur when the variable falls between the limits given above, or when the situation is unclear.

                         

                        It was clear from assessment using the cube model (figure 1) that high time demands could not be accepted if there were concurrent high or moderate demands in terms of force and postural strain. In order to reduce these demands, mechanized object handling and tool suspension was deemed a necessity. There was consensus developed around this conclusion. Using a simple computer-aided design (CAD) program (ROOMER), an equipment library was created. Various workplace station layouts could be developed very easily and modified in close interaction with the users. This design approach has significant advantages compared with merely looking at plans. It gives the user an immediate vision of what the intended workplace may look like.

                        Figure 2.  A CAD version of a workstation for manual welding, arrived at in the design process

                        ERG190F2

                        Figure 2 shows the welding workstation arrived at using the CAD system. It is a workplace which reduces the force and posture demands, and which meets nearly all the residual user demands put forward.

                         

                         

                         

                         

                         

                        Figure 3. The welding workstation implemented

                        ERG190F3

                        On the basis of the results of the first stages of the design process, a welding workplace (figure 3) was implemented. Assets of this workplace include:

                          1. Work in the optimized zone is facilitated using a computerized handling device for welding objects. There is an overhead hoist for transportation purposes. As an alternative, a balanced lifting device is supplied for easy object handling.
                          2. The welding gun and grinding machine are suspended, thus reducing force demands. They can be positioned anywhere around the welding object. A welding chair is supplied.
                          3. All media come from above, which means that there are no cables on the floor.
                          4. The workstation has lighting at three levels: general, workplace and process. The workplace lighting comes from ramps above the wall elements. The process lighting is integrated in the welding smoke ventilation arm.
                          5. The workstation has ventilation at three levels: general displacement ventilation, workplace ventilation using a movable arm, and integrated ventilation in the MIG welding gun. The workplace ventilation is controlled from the welding gun.
                          6. There are noise-absorbing wall elements on three sides of the workplace. A transparent welding curtain covers the fourth wall. This makes it possible for the welder to keep informed of what happens in the workshop environment.

                                     

                                    In a real design situation, compromises of various kinds may have to be made, due to economic, space and other constraints. It should be noted, however, that licensed welders are hard to come by for the welding industry around the world, and they represent a considerable investment. Nearly no welders go into normal retirement as active welders. Keeping the skilled welder on the job is beneficial for all parties involved: welder, company and society. For instance, there are very good reasons why equipment for object handling and positioning should be an integral constituent of many welding workplaces.

                                    Data for Workstation Design

                                    In order to be able to design a workplace properly, extensive sets of basic information may be needed. Such information includes anthropometric data of user categories, lifting strength and other output force capacity data of male and female populations, specifications of what constitutes optimal working zones and so forth. In the present article, references to some key papers are given.

                                    The most complete treatment of virtually all aspects of work and workstation design is probably still the textbook by Grandjean (1988). Information on a wide range of anthropometric aspects relevant to workstation design is presented by Pheasant (1986). Large amounts of biomechanical and anthropometric data are given by Chaffin and Andersson (1984). Konz (1990) has presented a practical guide to workstation design, including many useful rules of thumb. Evaluation criteria for the upper limb, particularly with reference to cumulative trauma disorders, have been presented by Putz-Anderson (1988). An assessment model for work with hand tools was given by Sperling et al. (1993). With respect to manual lifting, Waters and co-workers have developed the revised NIOSH equation, summarizing existing scientific knowledge on the subject (Waters et al. 1993). Specification of functional anthropometry and optimal working zones have been presented by, for example, Rebiffé, Zayana and Tarrière (1969) and Das and Grady (1983a, 1983b). Mital and Karwowski (1991) have edited a useful book reviewing various aspects relating in particular to the design of industrial workplaces.

                                    The large amount of data needed to design workstations properly, taking all relevant aspects into account, will make necessary the use of modern information technology by production engineers and other responsible people. It is likely that various types of decision-support systems will be made available in the near future, for instance in the form of knowledge-based or expert systems. Reports on such developments have been given by, for example, DeGreve and Ayoub (1987), Laurig and Rombach (1989), and Pham and Onder (1992). However, it is an extremely difficult task to devise a system making it possible for the end-user to have easy access to all relevant data needed in a specific design situation.

                                     

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                                    The entire topic of personal protection must be considered in the context of control methods for preventing occupational injuries and diseases. This article presents a detailed technical discussion of the types of personal protection which are available, the hazards for which their use may be indicated and the criteria for selecting appropriate protective equipment. Where they are applicable, the approvals, certifications and standards which exist for protective devices and equipment are summarized. In using this information, it is essential to be constantly mindful that personal protection should be considered the method of last resort in reducing the risks found in the workplace. In the hierarchy of methods which may be used to control workplace hazards, personal protection is not the method of first choice. In fact, it is to be used only when the possible engineering controls which reduce the hazard (by methods such as isolation, enclosure, ventilation, substitution, or other process changes), and administrative controls (such as reducing work time at risk for exposure) have been implemented to the extent feasible. There are cases, however, where personal protection is necessary, whether as a short-term or a long-term control, to reduce occupational disease and injury risks. When such use is necessary, personal protective equipment and devices must be used as part of a comprehensive programme which includes full evaluation of the hazards, correct selection and fitting of the equipment, training and education for the people who use the equipment, maintenance and repair to keep the equipment in good working order and overall management and worker commitment to the success of the protection programme.

                                    Elements of a Personal Protection Programme

                                    The apparent simplicity of some personal protective equipment can result in a gross underestimation of the amount of effort and expense required to effectively use this equipment. While some devices are relatively simple, such as gloves and protective footwear, other equipment such as respirators can actually be very complex. The factors which make effective personal protection difficult to achieve are inherent in any method which relies upon modification of human behaviour to reduce risk, rather than on protection which is built into the process at the source of the hazard. Regardless of the particular type of protective equipment being considered, there is a set of elements which must be included in a personal protection programme.

                                    Hazard evaluation

                                    If personal protection is to be an effective answer to a problem of occupational risk, the nature of the risk itself and its relationship to the overall work environment must be fully understood. While this may seem so obvious that it barely needs to be mentioned, the apparent simplicity of many protective devices can present a strong temptation to short cut this evaluation step. The consequences of providing protective devices and equipment which are not suitable to the hazards and the overall work environment range from reluctance or refusal to wear inappropriate equipment, to impaired job performance, to risk of worker injury and death. In order to achieve a proper match between the risk and the protective measure, it is necessary to know the composition and magnitude (concentration) of the hazards (including chemical, physical or biological agents), the length of time for which the device will be expected to perform at a known level of protection, and the nature of the physical activity which may be performed while the equipment is in use. This preliminary evaluation of the hazards is an essential diagnostic step which must be accomplished before moving on to selecting the appropriate protection.

                                    Selection

                                    The selection step is dictated in part by the information obtained in hazard evaluation, matched with the performance data for the protective measure being considered for use and the level of exposure which will remain after the personal protective measure is in place. In addition to these performance-based factors, there are guidelines and standards of practice in selecting equipment, particularly for respiratory protection. The selection criteria for respiratory protection have been formalized in publications such as Respirator Decision Logic from the National Institute for Occupational Safety and Health (NIOSH) in the United States. The same sort of logic can be applied to selecting other types of protective equipment and devices, based upon the nature and magnitude of the hazard, the degree of protection provided by the device or equipment, and the quantity or concentration of the hazardous agent which will remain and be considered acceptable while the protective devices are in use. In selecting protective devices and equipment, it is important to recognize that they are not intended to reduce risks and exposures to zero. Manufacturers of devices such as respirators and hearing protectors supply data on the performance of their equipment, such as protection and attenuation factors. By combining three essential pieces of information—namely, the nature and magnitude of the hazard, the degree of protection provided, and the acceptable level of exposure and risk while the protection is in use—equipment and devices can be selected to adequately protect workers.

                                    Fitting

                                    Any protective device must be properly fitted if it is to provide the degree of protection for which it was designed. In addition to the performance of a protective device, proper fit is also an important factor in the acceptance of the equipment and the motivation of people to actually use it. Protection which is ill-fitting or uncomfortable is unlikely to be used as intended. In the worst case, poorly fitted equipment such as clothing and gloves can actually create a hazard when working around machinery. Manufacturers of protective equipment and devices offer a range of sizes and designs of these products, and workers should be provided with protection which fits properly to accomplish its intended purpose.

                                    In the case of respiratory protection, specific requirements for fitting are included in standards such as the United States Occupational Safety and Health Administration’s respiratory protection standards. The principles of assuring proper fit apply over the full range of protective equipment and devices, regardless of whether they are required by a specific standard.

                                    Training and education

                                    Because the nature of protective devices requires modification of human behaviour to isolate the worker from the work environment (rather than to isolate the source of a hazard from the environment), personal protection programmes are unlikely to succeed unless they include comprehensive worker education and training. By comparison, a system (such as local exhaust ventilation) which controls exposure at the source may operate effectively without direct worker involvement. Personal protection, however, requires full participation and commitment by the people who use it and from the management which provides it.

                                    Those responsible for the management and operation of a personal protection programme must be trained in the selection of the proper equipment, in assuring that it is correctly fitted to the people who use it, in the nature of the hazards the equipment is intended to protect against, and the consequences of poor performance or equipment failure. They must also know how to repair, maintain, and clean the equipment, as well as to recognize damage and wear which occurs during its use.

                                    People who use protective equipment and devices must understand the need for the protection, the reasons it is being used in place of (or in addition to) other control methods, and the benefits they will derive from its use. The consequences of unprotected exposure should be clearly explained, as well as the ways users can recognize that the equipment is not functioning properly. Users must be trained in methods of inspecting, fitting, wearing, maintaining, and cleaning protective equipment, and they must also be aware of the limitations of the equipment, particularly in emergency situations.

                                    Maintenance and repair

                                    The costs of equipment maintenance and repair must be fully and realistically assessed in designing any personal protection programme. Protective devices are subject to gradual degradation in performance through normal use, as well as catastrophic failures in extreme conditions such as emergencies. In considering the costs and benefits of using personal protection as a means of hazard control it is very important to recognize that the costs of initiating a programme represent only a fraction of the total expense of operating the programme over time. Equipment maintenance, repair, and replacement must be considered as fixed costs of operating a programme, as they are essential to maintaining the effectiveness of protection. These programme considerations should include such basic decisions as whether single use (disposable) or reusable protective devices should be used, and in the case of reusable devices, the length of service which can be expected before replacement must be reasonably estimated. These decisions may be very clearly defined, as in cases where gloves or respirators are usable only once and are discarded, but in many cases a careful judgement must be made as to the efficacy of reusing protective suits or gloves which have been contaminated by previous use. The decision to discard an expensive protective device rather than risk worker exposure as a result of degraded protection, or contamination of the protective device itself must be made very carefully. Programmes of equipment maintenance and repair must be designed to include mechanisms for making decisions such as these.

                                    Summary

                                    Protective equipment and devices are essential parts of a hazard control strategy. They can be used effectively, provided their appropriate place in the hierarchy of controls is recognized. The use of protective equipment and devices must be supported by a personal protection programme, which assures that the protection actually performs as intended in conditions of use, and that the people who have to wear it can use it effectively in their work activities.

                                     

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                                    Monday, 14 March 2011 19:51

                                    Tools

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                                    Commonly a tool comprises a head and a handle, with sometimes a shaft, or, in the case of the power tool, a body. Since the tool must meet the requirements of multiple users, basic conflicts can arise which may have to be met with compromise. Some of these conflicts derive from limitations in the capacities of the user, and some are intrinsic to the tool itself. It should be remembered, however, that human limitations are inherent and largely immutable, while the form and function of the tool are subject to a certain amount of modification. Thus, in order to effect desirable change, attention must be directed primarily to the form of the tool, and, in particular, to the interface between the user and the tool, namely the handle.

                                    The Nature of Grip

                                    The widely accepted characteristics of grip have been defined in terms of a power grip, a precision grip and a hook grip, by which virtually all human manual activities can be accomplished.

                                    In a power grip, such as is used in hammering nails, the tool is held in a clamp formed by the partially flexed fingers and the palm, with counterpressure being applied by the thumb. In a precision grip, such as one uses when adjusting a set screw, the tool is pinched between the flexor aspects of the fingers and the opposing thumb. A modification of the precision grip is the pencil grip, which is self-explanatory and is used for intricate work. A precision grip provides only 20% of the strength of a power grip.

                                    A hook grip is used where there is no requirement for anything other than holding. In the hook grip the object is suspended from the flexed fingers, with or without the support of the thumb. Heavy tools should be designed so that they can be carried in a hook grip.

                                    Grip Thickness

                                    For precision grips, recommended thicknesses have varied from 8 to 16 millimetres (mm) for screwdrivers, and 13 to 30 mm for pens. For power grips applied around a more or less cylindrical object, the fingers should surround more than half the circumference, but the fingers and thumb should not meet. Recommended diameters have ranged from as low as 25 mm to as much as 85 mm. The optimum, varying with hand size, is probably around 55 to 65 mm for males, and 50 to 60 mm for females. Persons with small hands should not perform repetitive actions in power grips of diameter greater than 60 mm.

                                    Grip Strength and Hand Span

                                    The use of a tool requires strength. Other than for holding, the greatest requirement for hand strength is found in the use of cross-lever action tools such as pliers and crushing tools. The effective force in crushing is a function of the grip strength and the required span of the tool. The maximum functional span between the end of the thumb and the ends of the grasping fingers averages about 145 mm for men and 125 mm for women, with ethnic variations. For an optimal span, which ranges from 45 to 55 mm for both men and women, the grip strength available for a single short-term action ranges from about 450 to 500 newtons for men and 250 to 300 newtons for women, but for repetitive action the recommended requirement is probably closer to 90 to 100 newtons for men, and 50 to 60 newtons for women. Many commonly used clamps or pliers are beyond the capacity of one-handed use, particularly in women.

                                    When a handle is that of a screwdriver or similar tool the available torque is determined by the user’s ability to transmit force to the handle, and thus is determined by both the coefficient of friction between hand and handle and the diameter of the handle. Irregularities in the shape of the handle make little or no difference to the ability to apply torque, although sharp edges can cause discomfort and eventual tissue damage. The diameter of a cylindrical handle that allows the greatest application of torque is 50 to 65 mm, while that for a sphere is 65 to 75 mm.

                                    Handles

                                    Shape of handle

                                    The shape of a handle should maximize contact between skin and handle. It should be generalized and basic, commonly of flattened cylindrical or elliptical section, with long curves and flat planes, or a sector of a sphere, put together in such a manner as to conform to the general contours of the grasping hand. Because of its attachment to the body of a tool, the handle may also take the form of a stirrup, a T-shape or an L-shape, but the portion that contacts the hand will be in the basic form.

                                    The space enclosed by the fingers is, of course, complex. The use of simple curves is a compromise intended to meet the variations represented by different hands and different degrees of flexion. In this regard, it is undesirable to introduce any contour matching of flexed fingers into the handle in the form of ridges and valleys, flutings and indentations, since, in fact, these modifications would not fit a significant number of hands and might indeed, over a prolonged period, cause pressure injury to the soft tissues. In particular, recesses of greater that 3 mm are not recommended.

                                    A modification of the cylindrical section is the hexagonal section, which is of particular value in the design of small calibre tools or instruments. It is easier to maintain a stable grip on a hexagonal section of small calibre than on a cylinder. Triangular and square sections have also been used with varying degrees of success. In these cases, the edges must be rounded to avert pressure injury.

                                    Grip Surface and Texture

                                    It is not by accident that for millennia wood has been the material of choice for tool handles other than those for crushing tools like pliers or clamps. In addition to its aesthetic appeal, wood has been readily available and easily worked by unskilled workers, and has qualities of elasticity, thermal conductivity, frictional resistance and relative lightness in relation to bulk that have made it very acceptable for this and other uses.

                                    In recent years, metal and plastic handles have become more common for many tools, the latter in particular for use with light hammers or screwdrivers. A metal handle, however, transmits more force to the hand, and preferably should be encased in a rubber or plastic sheath. The grip surface should be slightly compressible, where feasible, nonconductive and smooth, and the surface area should be maximized to ensure pressure distribution over as large an area as possible. A foam rubber grip has been used to reduce the perception of hand fatigue and tenderness.

                                    The frictional characteristics of the tool surface vary with the pressure exerted by the hand, with the nature of the surface and contamination by oil or sweat. A small amount of sweat increases the coefficient of friction.

                                    Length of handle

                                    The length of the handle is determined by the critical dimensions of the hand and the nature of the tool. For a hammer to be used by one hand in a power grip, for example, the ideal length ranges from a minimum of about 100 mm to a maximum of about 125 mm. Short handles are unsuitable for a power grip, while a handle shorter than 19 mm cannot be properly grasped between thumb and forefinger and is unsuitable for any tool.

                                    Ideally, for a power tool, or a hand saw other than a coping or fret saw, the handle should accommodate at the 97.5th percentile level the width of the closed hand thrust into it, namely 90 to 100 mm in the long axis and 35 to 40 mm in the short.

                                    Weight and Balance

                                    Weight is not a problem with precision tools. For heavy hammers and power tools a weight between 0.9 kg and 1.5 kg is acceptable, with a maximum of about 2.3 kg. For weights greater than recommended, the tool should be supported by mechanical means.

                                    In the case of a percussion tool such as a hammer, it is desirable to reduce the weight of the handle to the minimum compatible with structural strength and have as much weight as possible in the head. In other tools, the balance should be evenly distributed where possible. In tools with small heads and bulky handles this may not be possible, but the handle should then be made progressively lighter as the bulk increases relative to the size of the head and shaft.

                                    Significance of Gloves

                                    It is sometimes overlooked by tool designers that tools are not always held and operated by bare hands. Gloves are commonly worn for safety and comfort. Safety gloves are seldom bulky, but gloves worn in cold climates may be very heavy, interfering not only with sensory feedback but also with the ability to grasp and hold. The wearing of woollen or leather gloves can add 5 mm to hand thickness and 8 mm to hand breadth at the thumb, while heavy mittens can add as much as 25 to 40 mm respectively.

                                    Handedness

                                    The majority of the population in the western hemisphere favours the use of the right hand. A few are functionally ambidextrous, and all persons can learn to operate with greater or less efficiency with either hand.

                                    Although the number of left-handed persons is small, wherever feasible the fitting of handles to tools should make the tool workable by either left-handed or right-handed persons (examples would include the positioning of the secondary handle in a power tool or the finger loops in scissors or clamps) unless it is clearly inefficient to do so, as in the case of screw-type fasteners which are designed to take advantage of the powerful supinating muscles of the forearm in a right-handed person while precluding the left-hander from using them with equal effectiveness. This sort of limitation has to be accepted since the provision of left-hand threads is not an acceptable solution.

                                    Significance of Gender

                                    In general, women tend to have smaller hand dimensions, smaller grasp and some 50 to 70% less strength than men, although of course a few women at the higher percentile end have larger hands and greater strength than some men at the lower percentile end. As a result there exists a significant although undetermined number of persons, mostly female, who have difficulty in manipulating various hand tools which have been designed with male use in mind, including in particular heavy hammers and heavy pliers, as well as metal cutting, crimping and clamping tools and wire strippers. The use of these tools by women may require an undesirable two-handed instead of single-handed function. In a mixed-gender workplace it is therefore essential to ensure that tools of suitable size are available not only to meet the requirements of women, but also to meet those of men who are at the low percentile end of hand dimensions.

                                    Special considerations

                                    The orientation of a tool handle, where feasible, should allow the operating hand to conform to the natural functional position of the arm and hand, namely with the wrist more than half-supinated, abducted about 15° and slightly dorsiflexed, with the little finger in almost full flexion, the others less so and the thumb adducted and slightly flexed, a posture sometimes erroneously called the handshake position. (In a handshake the wrist is not more than half-supinated.) The combination of adduction and dorsiflexion at the wrist with varying flexion of the fingers and thumb generates an angle of grasp comprising about 80° between the long axis of the arm and a line passing through the centre point of the loop created by the thumb and index finger, that is, the transverse axis of the fist.

                                    Forcing the hand into a position of ulnar deviation, that is, with the hand bent towards the little finger, as is found in using a standard pliers, generates pressure on the tendons, nerves and blood vessels within the wrist structure and can give rise to the disabling conditions of tenosynovitis, carpal tunnel syndrome and the like. By bending the handle and keeping the wrist straight, (that is, by bending the tool and not the hand) compression of nerves, soft tissues and blood vessels can be avoided. While this principle has been long recognized, it has not been widely accepted by tool manufacturers or the using public. It has particular application in the design of cross-lever action tools such as pliers, as well as knives and hammers.

                                    Pliers and cross-lever tools

                                    Special consideration must be given to the shape of the handles of pliers and similar devices. Traditionally pliers have had curved handles of equal length, the upper curve approximating the curve of the palm of the hand and the lower curve approximating the curve of the flexed fingers. When the tool is held in the hand, the axis between the handles is in line with the axis of the jaws of the pliers. Consequently, in operation, it is necessary to hold the wrist in extreme ulnar deviation, that is, bent towards the little finger, while it is being repeatedly rotated. In this position the use of the hand-wrist-arm segment of the body is extremely inefficient and very stressful on the tendons and joint structures. If the action is repetitive it may give rise to various manifestations of overuse injury.

                                    To counter this problem a new and ergonomically more suitable version of pliers has appeared in recent years. In these pliers the axis of the handles is bent through approximately 45° relative to the axis of the jaws. The handles are thickened to allow a better grasp with less localized pressure on the soft tissues. The upper handle is proportionately longer with a shape that fits into, and around the ulnar side of, the palm. The forward end of the handle incorporates a thumb support. The lower handle is shorter, with a tang, or rounded projection, at the forward end and a curve conforming to the flexed fingers.

                                    While the foregoing is a somewhat radical change, several ergonomically sound improvements can be made in pliers relatively easily. Perhaps the most important, where a power grip is required, is in the thickening and slight flattening of the handles, with a thumb support at the head-end of the handle and a slight flare at the other end. If not integral to the design, this modification can be achieved by encasing the basic metal handle with a fixed or detachable non-conductive sheath made of rubber or an appropriate synthetic material, and perhaps bluntly roughened to improve the tactile quality. Indentation of the handles for fingers is undesirable. For repetitive use it may be desirable to incorporate a light spring into the handle to open it after closing.

                                    The same principles apply to other cross-lever tools, particularly with respect to change in the thickness and flattening of the handles.

                                    Knives

                                    For a general purpose knife, that is, one that is not used in a dagger grasp, it is desirable to include a 15° angle between handle and blade to reduce the stress on joint tissues. The size and shape of handles should conform in general to that for other tools, but to allow for different hand sizes it has been suggested that two sizes of knife handle should be supplied, namely one to fit the 50th to 95th percentile user, and one for the 5th to 50th percentile. To allow the hand to exert force as close to the blade as possible the top surface of the handle should incorporate a raised thumb rest.

                                    A knife guard is required to prevent the hand from slipping forward onto the blade. The guard may take several forms, such as a tang, or curved projection, about 10 to 15 mm in length, protruding downwards from the handle, or at right angles to the handle, or a bail guard comprising a heavy metal loop from front to rear of the handle. The thumb rest also acts to prevent slippage.

                                    The handle should conform to general ergonomic guidelines, with a yielding surface resistant to grease.

                                    Hammers

                                    The requirements for hammers have been largely considered above, with the exception of that relating to bending the handle. As noted above, forced and repetitive bending of the wrist may cause tissue damage. By bending the tool instead of the wrist this damage may be reduced. With respect to hammers various angles have been examined, but it would appear that bending the head downward between 10° and 20° may improve comfort, if it does not actually improve performance.

                                    Screwdrivers and scraping tools

                                    The handles of screwdrivers and other tools held in a somewhat similar manner, such as scrapers, files, hand chisels and so on, have some special requirements. Each at one time or another is used with a precision grip or a power grip. Each relies on the functions of the fingers and the palm of the hand for stabilization and the transmission of force.

                                    The general requirements of handles have already been considered. The most common effective shape of a screwdriver handle has been found to be that of a modified cylinder, dome-shaped at the end to receive the palm, and slightly flared where it meets the shaft to provide support to the ends of the fingers. In this manner, torque is applied largely by way of the palm, which is maintained in contact with the handle by way of pressure applied from the arm and the frictional resistance at the skin. The fingers, although transmitting some force, occupy more of a stabilizing role, which is less fatiguing since less power is required. Thus the dome of the head becomes very important in handle design. If there are sharp edges or ridges on the dome or where the dome meets the handle, then either the hand becomes callused and injured, or the transmission of force is transferred towards the less efficient and more readily fatigued fingers and thumb. The shaft is commonly cylindrical, but a triangular shaft has been introduced which provides better support for the fingers, although its use may be more fatiguing.

                                    Where the use of a screwdriver or other fastener is so repetitive as to comprise an overuse injury hazard the manual driver should be replaced with a powered driver slung from an overhead harness in such a manner as to be readily accessible without obstructing the work.

                                    Saws and power tools

                                    Hand saws, with the exception of fret saws and light hacksaws, where a handle like that of a screwdriver is most appropriate, commonly have a handle which takes the form of a closed pistol grip attached to the blade of the saw.

                                    The handle essentially comprises a loop into which the fingers are placed. The loop is effectively a rectangle with curved ends. To allow for gloves it should have internal dimensions of approximately 90 to 100 mm in the long diameter and 35 to 40 mm in the short. The handle in contact with the palm should have the flattened cylindrical shape already mentioned, with compound curves to reasonably fit the palm and the flexed fingers. The width from outer curve to inner curve should be about 35 mm, and the thickness not more than 25 mm.

                                    Curiously, the function of grasping and holding a power tool is very similar to that of holding a saw, and consequently a somewhat similar type of handle is effective. The pistol grip common in power tools is akin to an open saw handle with the sides being curved instead of being flattened.

                                    Most power tools comprise a handle, a body and a head. Placement of the handle is significant. Ideally handle, body and head should be in line so that the handle is attached at the rear of the body and the head protrudes from the front. The line of action is the line of the extended index finger, so that the head is eccentric to the central axis of the body. The centre of mass of the tool, however, is in front of the handle, while the torque is such as to create a turning movement of the body which the hand must overcome. Consequently it would be more appropriate to place the primary handle directly under the centre of mass in such a way that, if necessary, the body juts out behind the handle as well as in front. Alternatively, particularly in a heavy drill, a secondary handle can be placed underneath the drill in such a manner that the drill can be operated with either hand. Power tools are normally operated by a trigger incorporated into the upper front end of the handle and operated by the index finger. The trigger should be designed to be operated by either hand and should incorporate an easily reset latching mechanism to hold the power on when required.

                                     

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                                    Thursday, 17 March 2011 15:51

                                    Eye and Face Protections

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                                    Eye and face protection includes safety spectacles, goggles, face shields and similar items used to protect against flying particles and foreign bodies, corrosive chemicals, fumes, lasers and radiation. Often, the whole face may need protection against radiation or mechanical, thermal or chemical hazards. Sometimes a face shield may be adequate also for protecting the eyes, but often specific eye protection is necessary, either separately or as a complement to the face protection.

                                    A wide range of occupations require eye and face protectors: hazards include flying particles, fumes or corrosive solids, liquids or vapours in polishing, grinding, cutting, blasting, crushing, galvanizing or various chemical operations; against intensive light as in laser operations; and against ultraviolet or infrared radiation in welding or furnace operations. Of the many types of eye and face protection available, there is a correct type for each hazard. Whole-face protection is preferred for certain severe risks. As needed, hood or helmet type face protectors and face shields are used. Spectacles or goggles may be used for specific eye protection.

                                    The two basic problems in wearing eye and face protectors are (1) how to provide effective protection which is acceptable for wearing over long hours of work without undue discomfort, and (2) the unpopularity of eye and face protection due to restriction of vision. The wearer’s peripheral vision is limited by the side frames; the nose bridge may disturb binocular vision; and misting is a constant problem. Particularly in hot climates or in hot work, additional coverings of the face may become intolerable and may be discarded. Short-term, intermittent operations also create problems as workers may be forgetful and disinclined to use protection. First consideration should always be given to the improvement of the working environment rather than to the possible need for personal protection. Before or in conjunction with the use of eye and face protection, consideration must be given to guarding of machines and tools (including interlocking guards), removal of fumes and dust by exhaust ventilation, screening of sources of heat or radiation, and screening of points from which particles may be ejected, such as abrasive grinders or lathes. When the eyes and face can be protected by the use of transparent screens or partitions of appropriate size and quality, for example, these alternatives are to be preferred to the use of personal eye protection.

                                    There are six basic types of eye and face protection:

                                    1. spectacle type, either with or without side shields (figure 1)
                                    2. eye cup (goggle) type (figure 2)
                                    3. face shield type, covering eye sockets and the central portion of the face (figure 3)
                                    4. helmet type with shielding of the whole front of the face (figure 4)
                                    5. hand-held shield type (see figure 4)
                                    6. hood type, including the diver’s helmet type covering the head completely (see figure 4)

                                    Figure 1. Common types of spectacles for eye protection with or without sideshield

                                    PPE020F1

                                    Figure 2. Examples of goggle-type eye protectors

                                    PPE020F2.

                                    Figure 3. Face shield type protectors for hot work

                                    PPE020F3

                                    Figure 4. Protectors for welders

                                    PPE020F4

                                    There are goggles that may be worn over corrective spectacles. It is often better for the hardened lenses of such goggles to be fitted under the guidance of an ophthalmic specialist.

                                    Protection against Specific Hazards

                                    Traumatic and chemical injuries. Face shields or eye protectors are used against flying
                                    particles, fumes, dust and chemical hazards. Common types are spectacles (often with side shields), goggles, plastic eye shields and face shields. The helmet type is used when injury risks are expected from various directions. The hood type and the diver’s helmet type are used in sand- and shot-blasting. Transparent plastics of various sorts, hardened glass or a wire screen may be used for protection against certain foreign bodies. Eye cup goggles with plastic or glass lenses or plastic eye shields as well as a diver’s helmet type shield or face shields made of plastic are used for protection against chemicals.

                                    Materials commonly used include polycarbonates, acrylic resins or fibre-based plastics. Polycarbonates are effective against impacts but may not be suitable against corrosives. Acrylic protectors are weaker against impacts but suitable for protection from chemical hazards. Fibre-based plastics have the advantage of adding anti-misting coating. This anti-misting coating also prevents electrostatic effects. Thus such plastic protectors may be used not only in physically light work or chemical handling but also in modern clean-room work.

                                    Thermal radiation. Face shields or eye protectors against infrared radiation are used mainly in furnace operations and other hot work involving exposure to high-temperature radiation sources. Protection is usually necessary at the same time against sparks or flying hot objects. Face protectors of the helmet type and the face shield type are mainly used. Various materials are used, including metal wire meshes, punched aluminium plates or similar metal plates, aluminized plastic shields or plastic shields with gold layer coatings. A face shield made of wire mesh can reduce thermal radiation by 30 to 50%. Aluminized plastic shields give good protection from radiant heat. Some examples of face shields against thermal radiation are given in figure 1.

                                    Welding. Goggles, helmets or shields that give maximum eye protection for each welding and cutting process should be worn by operators, welders and their helpers. Effective protection is needed not only against intensive light and radiation but also against impacts upon the face, head and neck. Fibreglass-reinforced plastic or nylon protectors are effective but rather expensive. Vulcanized fibres are commonly used as shield material. As shown in figure 4, both helmet type protectors and hand-held shields are used to protect the eyes and face at the same time. Requirements for correct filter lenses to be used in various welding and cutting operations are described below.

                                    Wide spectral bands. Welding and cutting processes or furnaces emit radiations in the ultraviolet, visible and infrared bands of the spectrum, which are all able to produce harmful effects upon the eyes. Spectacle type or goggle type protectors similar to those shown in figure 1 and figure 2 as well as welders’ protectors such as those shown in figure 4 can be used. In welding operations, helmet type protection and hand-shield type protectors are generally used, sometimes in conjunction with spectacles or goggles. It should be noted that protection is necessary also for the welder’s assistant.

                                    Transmittance and tolerances in transmittance of various shades of filter lenses and filter plates of eye protection against high-intensity light are shown in table 1. Guides for selecting correct filter lenses in terms of the scales of protection are given in table 2 through table 6).

                                     


                                    Table 1. Transmittance requirements (ISO 4850-1979)

                                     

                                    Scale number

                                    Maximum transmittance

                                    in the ultraviolet spectrum t (), %

                                    Luminous transmittance ( ), %

                                    Maximum mean transmittance

                                    in the infrared spectrum , %

                                    313 nm

                                    365 nm

                                    maximum

                                    minimum

                                    Near IR

                                    1,300 to 780 nm,

                                    Mid. IR

                                    2,000 to 1,300 nm ,

                                    1.2

                                    1.4

                                    1.7

                                    2.0

                                    2.5

                                    3

                                    4

                                    5

                                    6

                                    7

                                    8

                                    9

                                    10

                                    11

                                    12

                                    13

                                    14

                                    15

                                    16

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    0,0003

                                    Value less than or equal to transmittance  permitted for   365 nm

                                    50

                                    35

                                    22

                                    14

                                    6,4

                                    2,8

                                    0,95

                                    0,30

                                    0,10

                                    0,037

                                    0,013

                                    0,0045

                                    0,0016

                                    0,00060

                                    0,00020

                                    0,000076

                                    0,000027

                                    0,0000094

                                    0,0000034

                                    100

                                    74,4

                                    58,1

                                    43,2

                                    29,1

                                    17,8

                                    8,5

                                    3,2

                                    1,2

                                    0,44

                                    0,16

                                    0,061

                                    0,023

                                    0,0085

                                    0,0032

                                    0,0012

                                    0,00044

                                    0,00016

                                    0,000061

                                    74,4

                                    58,1

                                    43,2

                                    29,1

                                    17,8

                                    8,5

                                    3,2

                                    1,2

                                    0,44

                                    0,16

                                    0,061

                                    0,023

                                    0,0085

                                    0,0032

                                    0,0012

                                    0,00044

                                    0,00016

                                    0,000061

                                    0,000029

                                    37

                                    33

                                    26

                                    21

                                    15

                                    12

                                    6,4

                                    3,2

                                    1,7

                                    0,81

                                    0,43

                                    0,20

                                    0,10

                                    0,050

                                    0,027

                                    0,014

                                    0,007

                                    0,003

                                    0,003

                                    37

                                    33

                                    26

                                    13

                                    9,6

                                    8,5

                                    5,4

                                    3,2

                                    1,9

                                    1,2

                                    0,68

                                    0,39

                                    0,25

                                    0,15

                                    0,096

                                    0,060

                                    0,04

                                    0,02

                                    0,02

                                    Taken from ISO 4850:1979 and reproduced with the permission of the International Organization for Standardization (ISO). These standards can be obtained from any ISO member or from the ISO Central Secretariat, Case postale 56, 1211 Geneva 20, Switzerland. Copyright remains with ISO.


                                     

                                    Table 2. Scales of protection to be used for gas-welding and braze-welding

                                    Work to be carried out1

                                    l = flow rate of acetylene, in litres per hour

                                     

                                    l £ 70

                                    70 < l £ 200

                                    200 < l £ 800

                                    l > 800

                                    Welding and braze-welding
                                    of heavy metals

                                    4

                                    5

                                    6

                                    7

                                    Welding with emittive
                                    fluxes (notably light alloys)

                                    4a

                                    5a

                                    6a

                                    7a

                                    1 According to the conditions of use, the next greater or the next smaller scale can be used.

                                    Taken from ISO 4850:1979 and reproduced with the permission of the International Organization for Standardization (ISO). These standards can be obtained from any ISO member or from the ISO Central Secretariat, Case postale 56, 1211 Geneva 20, Switzerland. Copyright remains with ISO.


                                     

                                    Table 3. Scales of protection to be used for oxygen cutting

                                    Work to be carried out1

                                    Flow rate of oxygen, in litres per hour

                                     

                                    900 to 2,000

                                    2,000 to 4,000

                                    4,000 to 8,000

                                    Oxygen cutting

                                    5

                                    6

                                    7

                                    1 According to the conditions of use, the next greater or the next smaller scale can be used.

                                    NOTE: 900 to 2,000 and 2,000 to 8,000 litres of oxygen per hour, correspond fairly closely to the use of cutting nozzles diameters of 1 to 1.5 and 2 mm respectively.

                                    Taken from ISO 4850:1979 and reproduced with the permission of the International Organization for Standardization (ISO). These standards can be obtained from any ISO member or from the ISO Central Secretariat, Case postale 56, 1211 Geneva 20, Switzerland. Copyright remains with ISO.


                                     

                                    Table 4. Scales of protection to be used for plasma arc cutting

                                    Work to be carried out1

                                    l = Current, in amperes

                                     

                                    l £ 150

                                    150 < l £ 250

                                    250 < l £ 400

                                    Thermal cutting

                                    11

                                    12

                                    13

                                    1 According to the conditions of use, the next greater or the next smaller scale can be used.

                                    Taken from ISO 4850:1979 and reproduced with the permission of the International Organization for Standardization (ISO). These standards can be obtained from any ISO member or from the ISO Central Secretariat, Case postale 56, 1211 Geneva 20, Switzerland. Copyright remains with ISO.


                                     

                                    Table 5. Scales of protection to be used for electric arc welding or gouging

                                    1 According to the conditions of use, the next greater or the next smaller scale can be used.

                                    2 The expression “heavy metals” applies to steels, alloy stells, copper and its alloys, etc.

                                    NOTE: The coloured areas correspond to the ranges where the welding operations are not usually used in the current practice of manual welding.

                                    Taken from ISO 4850:1979 and reproduced with the permission of the International Organization for Standardization (ISO). These standards can be obtained from any ISO member or from the ISO Central Secretariat, Case postale 56, 1211 Geneva 20, Switzerland. Copyright remains with ISO.


                                     

                                    Table 6. Scales of protection to be used for plasma direct arc welding

                                    1 According to the conditions of use, the next greater or the next smaller scale can be used.

                                    The coloured areas correspond to the ranges where the welding operations are not usually used in the current practice of manual welding.

                                    Taken from ISO 4850:1979 and reproduced with the permission of the International Organization for Standardization (ISO). These standards can be obtained from any ISO member or from the ISO Central Secretariat, Case postale 56, 1211 Geneva 20, Switzerland. Copyright remains with ISO.


                                     

                                    A new development is the use of filter plates made of welded crystal surfaces which increase their protective shade as soon as the welding arc starts. The time for this nearly instantaneous shade increase can be as short as 0.1 ms. The good visibility through the plates in non-welding situations can encourage their use.

                                    Laser beams. No one type of filter offers protection from all laser wavelengths. Different kinds of lasers vary in wavelength, and there are lasers that produce beams of various wavelengths or those whose beams change their wavelengths by passing through optical systems. Consequently, laser-using firms should not depend solely on laser protectors to protect an employee’s eyes from laser burns. Nevertheless, laser operators do frequently need eye protection. Both spectacles and goggles are available; they have shapes similar to those shown in figure 1 and figure 2. Each kind of eyewear has maximum attenuation at a specific laser wavelength. Protection falls off rapidly at other wavelengths. It is essential to select the correct eyewear appropriate for the kind of laser, its wavelength and optical density. The eyewear is to provide protection from reflections and scattered lights and the utmost precautions are necessary to foresee and avoid harmful radiation exposure.

                                    With the use of eye and face protectors, due attention must be paid to greater comfort and efficiency. It is important that the protectors be fitted and adjusted by a person who has received some training in this task. Each worker should have the exclusive use of his or her own protector, while communal provision for cleaning and demisting may well be made in larger works. Comfort is particularly important in helmet and hood type protectors as they may become almost intolerably hot during use. Air lines can be fitted to prevent this. Where the risks of the work process allow, some personal choice among different types of protection is psychologically desirable.

                                    The protectors should be examined regularly to ensure that they are in good condition. Care should be taken that they give adequate protection at all times even with the use of corrective vision devices.

                                     

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                                    Monday, 14 March 2011 19:54

                                    Controls, Indicators and Panels

                                    Written by

                                    Karl H. E. Kroemer

                                    In what follows, three of the most important concerns of ergonomic design will be examined: first, that of controls, devices to transfer energy or signals from the operator to a piece of machinery; second, indicators or displays, which provide visual information to the operator about the status of the machinery; and third, the combination of controls and displays in a panel or console.

                                    Designing for the Sitting Operator

                                    Sitting is a more stable and less energy-consuming posture than standing, but it restricts the working space, particularly of the feet, more than standing. However, it is much easier to operate foot controls when sitting, as compared to standing, because little body weight must be transferred by the feet to the ground. Furthermore, if the direction of the force exerted by the foot is partly or largely forward, provision of a seat with a backrest allows the exertion of rather large forces. (A typical example of this arrangement is the location of pedals in an automobile, which are located in front of the driver, more or less below seat height.) Figure 1 shows schematically the locations in which pedals may be located for a seated operator. Note that the specific dimensions of that space depend on the anthropometry of the actual operators.

                                    Figure 1. Preferred and regular workspace for feet (in centimetres)

                                    ERG210F1

                                    The space for the positioning of hand-operated controls is primarily located in front of the body, within a roughly spherical contour that is centred at either the elbow, at the shoulder, or somewhere between those two body joints. Figure 2 shows schematically that space for the location of controls. Of course, the specific dimensions depend on the anthropometry of the operators.

                                     

                                    Figure 2. Preferred and regular workspace for hands (in centimetres)

                                    ERG210F2

                                    The space for displays and for controls that must be looked at is bounded by the periphery of a partial sphere in front of the eyes and centred at the eyes. Thus, the reference height for such displays and controls depends on the eye height of the seated operator and on his or her trunk and neck postures. The preferred location for visual targets closer than about one metre is distinctly below the height of the eye, and depends on the closeness of the target and on the posture of the head. The closer the target, the lower it should be located, and it should be in or near the medial (mid-sagittal) plane of the operator.

                                    It is convenient to describe the posture of the head by using the “ear-eye line” (Kroemer 1994a) which, in the side view, runs through the right ear hole and the juncture of the lids of the right eye, while the head is not tilted to either side (the pupils are at the same horizontal level in the frontal view). One usually calls the head position “erect” or “upright” when the pitch angle P (see figure 3) between the ear-eye line and the horizon is about 15°, with the eyes above the height of the ear. The preferred location for visual targets is 25°–65° below the ear-eye line (LOSEE in figure 3), with the lower values preferred by most people for close targets that must be kept in focus. Even though there are large variations in the preferred angles of the line of sight, most subjects, particularly as they become older, prefer to focus on close targets with large LOSEE angles.

                                    Figure 3. Ear-eye line

                                    ERG210F3

                                    Designing for the Standing Operator

                                    Pedal operation by a standing operator should be seldom required, because otherwise the person must spend too much time standing on one foot while the other foot operates the control. Obviously, simultaneous operation of two pedals by a standing operator is practically impossible. While the operator is standing still, the room for the location of foot controls is limited to a small area below the trunk and slightly in front of it. Walking about would provide more room to place pedals, but that is highly impractical in most cases because of the walking distances involved.

                                    The location for hand-operated controls of a standing operator includes about the same area as for a seated operator, roughly a half sphere in front of the body, with its centre near the shoulders of the operator. For repeated control operations, the preferred part of that half sphere would be its lower section. The area for the location of displays is also similar to the one suited to a seated operator, again roughly a half sphere centred near the operator’s eyes, with the preferred locations in the lower section of that half sphere. The exact locations for displays, and also for controls that must be seen, depends on the posture of the head, as discussed above.

                                    The height of controls is appropriately referenced to the height of the elbow of the operator while the upper arm is hanging from the shoulder. The height of displays and controls that must be looked at is referred to the eye height of the operator. Both depend on the operator’s anthropometry, which may be rather different for short and tall persons, for men and women, and for people of different ethnic origins.

                                    Foot-operated Controls

                                    Two kinds of controls should be distinguished: one is used to transfer large energy or forces to a piece of machinery. Examples of this are the pedals on a bicycle or the brake pedal in a heavier vehicle that does not have a power-assist feature. A foot-operated control, such as an on-off switch, in which a control signal is conveyed to the machinery, usually requires only a small quantity of force or energy. While it is convenient to consider these two extremes of pedals, there are various intermediate forms, and it is the task of the designer to determine which of the following design recommendations apply best among them.

                                    As mentioned above, repeated or continual pedal operation should be required only from a seated operator. For controls meant to transmit large energies and forces, the following rules apply:

                                    • Locate pedals underneath the body, slightly in front, so that they can be operated with the leg in a comfortable position. The total horizontal displacement of a reciprocating pedal should normally not exceed about 0.15 m. For rotating pedals, the radius should also be about 0.15 m. The linear displacement of a switch-type pedal may be minimal and should not exceed about 0.15 m.
                                    • Pedals should be so designed that the direction of travel and the foot force are approximately in the line extending from the hip through the ankle joint of the operator.
                                    • Pedals that are operated by flexion and extension of the foot in the ankle joint should be so arranged that in the normal position the angle between the lower leg and the foot is approximately 90°; during operation, that angle may be increased to about 120°.
                                    • Foot-operated controls that simply provide signals to the machinery should normally have two discrete positions, such as ON or OFF. Note, however, that tactile distinction between the two positions may be difficult with the foot.

                                     

                                    Selection of Controls

                                    Selection among different sorts of controls must be made according to the following needs or conditions:

                                    • Operation by hand or foot
                                    • Amounts of energies and forces transmitted
                                    • Applying “continuous” inputs, such as steering an automobile
                                    • Performing “discrete actions,” for example, (a) activating or shutting down equipment, (b) selecting one of several distinct adjustments, such as switching from one TV or radio channel to another, or (c) carrying out data entry, as with a keyboard.

                                     

                                    The functional usefulness of controls also determines selection procedures. The main criteria are as follows:

                                    • The control type shall be compatible with stereotypical or common expectations (for instance, using a push-button or toggle switch to turn on an electric light, not a rotary knob).
                                    • Size and motion characteristics of the control shall be compatible with stereotypical experience and past practice (for instance, providing a large steering wheel for the two-handed operation of an automobile, not a lever).
                                    • The direction of operation of a control shall be compatible with stereotypical or common expectations (for instance, an ON control is pushed or pulled, not turned to the left).
                                    • Hand operation is used for controls that require small force and fine adjustment, while foot operation is suitable for gross adjustments and large forces (however, consider the common use of pedals, particularly accelerator pedals, in automobiles, which does not comply with this principle).
                                    • The control shall be “safe” in that it cannot be operated inadvertently nor in ways that are excessive or inconsistent with its intended purpose.

                                     

                                    Table 1. Control movements and expected effects

                                    Direction of control movement

                                    Function

                                    Up

                                    Right

                                    Forward

                                    Clockwise

                                    Press,
                                    Squeeze

                                    Down

                                    Left

                                    Rearward

                                    Back

                                    Counter-
                                    clockwise

                                    Pull1

                                    Push2

                                    On

                                    +3

                                    +

                                    +

                                    +

                                    +3

                                           

                                    +

                                     

                                    Off

                                             

                                    +

                                     

                                    +

                                     

                                    Right

                                     

                                    +

                                     

                                                   

                                    Left

                                               

                                    +

                                     

                                         

                                    Raise

                                    +

                                               

                                           

                                    Lower

                                       

                                       

                                    +

                                               

                                    Retract

                                               

                                    +

                                       

                                     

                                    Extend

                                       

                                    +

                                       

                                             

                                    Increase

                                    +

                                                   

                                    Decrease

                                             

                                    +

                                     

                                       

                                    Open Value

                                             

                                         

                                    +

                                       

                                    Close Value

                                         

                                    +

                                     

                                               

                                    Blank: Not applicable; + Most preferred; – less preferred. 1 With trigger-type control. 2 With push-pull switch. 3 Up in the United States, down in Europe.

                                    Source: Modified from Kroemer 1995.

                                     

                                    Table 1 and table 2 help in the selection of proper controls. However, note that there are few “natural” rules for selection and design of controls. Most current recommendations are purely empirical and apply to existing devices and Western stereotypes.

                                    Table 2. Control-effect relations of common hand controls

                                    Effect

                                    Key-
                                    lock

                                    Toggle
                                    switch

                                    Push-
                                    button

                                    Bar
                                    knob

                                    Round
                                    knob

                                    Thumbwheel
                                    discrete

                                    Thumbwheel
                                    continuous

                                    Crank

                                    Rocker switch

                                    Lever

                                    Joystick
                                    or ball

                                    Legend
                                    switch

                                    Slide1

                                    Select ON/OFF

                                    +

                                    +

                                    +

                                    =

                                           

                                    +

                                       

                                    +

                                    +

                                    Select ON/STANDBY/OFF

                                     

                                    +

                                    +

                                             

                                    +

                                     

                                    +

                                    +

                                    Select OFF/MODE1/MODE2

                                     

                                    =

                                    +

                                             

                                    +

                                     

                                    +

                                    +

                                    Select one function of several  related functions

                                     

                                    +

                                             

                                         

                                    =

                                    Select one of three or more  discrete alternatives

                                         

                                    +

                                                   

                                    +

                                    Select operating condition

                                     

                                    +

                                    +

                                           

                                    +

                                    +

                                       

                                    Engage or disengage

                                                     

                                    +

                                         

                                    Select one of mutually
                                    exclusive functions

                                       

                                    +

                                                   

                                    +

                                     

                                    Set value on scale

                                           

                                    +

                                     

                                    =

                                     

                                    =

                                    =

                                     

                                    +

                                    Select value in discrete steps

                                       

                                    +

                                    +

                                     

                                    +

                                               

                                    +

                                    Blank: Not applicable; +: Most preferred; –: Less preferred; = Least preferred. 1 Estimated (no experiments known).

                                    Source: Modified from Kroemer 1995.

                                     

                                    Figure 4 presents examples of “detent” controls, characterized by discrete detents or stops in which the control comes to rest. It also depicts typical “continuous” controls where the control operation may take place anywhere within the adjustment range, without the need to be set in any given position.

                                    Figure 4. Some examples of "detent" and "continuous" controls

                                    ERG210F4

                                    The sizing of controls is largely a matter of past experiences with various control types, often guided by the desire to minimize the needed space in a control panel, and either to allow simultaneous operations of adjacent controls or to avoid inadvertent concurrent activation. Furthermore, the choice of design characteristics will be influenced by such considerations as whether the controls are to be located outdoors or in sheltered environments, in stationary equipment or moving vehicles, or may involve the use of bare hands or of gloves and mittens. For these conditions, consult readings at the end of the chapter.

                                    Several operational rules govern the arrangement and grouping of controls. These are listed in table 3. For more details, check the references listed at the end of this section and Kroemer, Kroemer and Kroemer-Elbert (1994).

                                    Table 3. Rules for arrangement of controls

                                    Locate for the
                                    ease of
                                    operation

                                    Controls shall be oriented with respect to the operator. If the
                                    operator uses different postures (such as in driving and
                                    operating a backhoe), the controls and their associated
                                    displays shall move with the operator so that in each posture
                                    their arrangement and operation is the same for the operator.

                                    Primary controls
                                    first

                                    The most important controls shall have the most advantageous
                                    locations to make operation and reaching easy for the
                                    operator.

                                    Group related
                                    controls
                                    together

                                    Controls that are operated in sequence, that are related to a
                                    particular function, or that are operated together, shall be
                                    arranged in functional groups (together with their associated
                                    displays). Within each functional group, controls and displays
                                    shall be arranged according to operational importance and
                                    sequence.

                                    Arrange for
                                    sequential
                                    operation

                                    If operation of controls follows a given pattern, controls shall
                                    be arranged to facilitate that sequence. Common
                                    arrangements are left-to-right (preferred) or top-to-bottom,
                                    as in printed materials of the Western world.

                                    Be consistent

                                    The arrangement of functionally identical or similar controls
                                    shall be the same from panel to panel.

                                    Dead-operator
                                    control

                                    If the operator becomes incapacitated and either lets go of a
                                    control, or continues to hold on to it, a “deadman” control
                                    design shall be utilized which either turns the system to a
                                    non-critical operation state or shuts it down.

                                    Select codes
                                    appropriately

                                    There are numerous ways to help identify controls, to indicate
                                    the effects of the operation and to show their status.
                                    Major coding means are:
                                    –Location–Shape–Size–Mode of operation– Labels
                                    –Colours–Redundancy

                                    Source: Modified from Kroemer, Kroemer and Kroemer-Elbert 1994.
                                    Reproduced by permission of Prentice-Hall. All rights reserved.

                                    Preventing Accidental Operation

                                    The following are the most important means to guard against inadvertent activation of controls, some of which may be combined:

                                    • Locate and orient the control so that the operator is unlikely to strike it or move it accidentally in the normal sequence of control operations.
                                    • Recess, shield or surround the control by physical barriers.
                                    • Cover the control or guard it by providing a pin, a lock or other means that must be removed or broken before the control can be operated.
                                    • Provide extra resistance (by viscous or coulomb friction, by spring-loading or by inertia) so that an unusual effort is required for actuation.
                                    • Provide a “delaying” means so that the control must pass through a critical position with an unusual movement (such as in the gear shift mechanism of an automobile).
                                    • Provide interlocking between controls so that prior operation of a related control is required before the critical control can be activated.

                                     

                                    Note that these designs usually slow the operation of controls, which may be detrimental in case of an emergency.

                                    Data Entry Devices

                                    Nearly all controls can be used to enter data on a computer or other data storage device. However, we are most used to the practice of using a keyboard with push-buttons. On the original typewriter keyboard, which has become the standard even for computer keyboards, the keys were arranged in a basically alphabetic sequence, which has been modified for various, often obscure, reasons. In some cases, letters which frequently follow each other in common text were spaced apart so that the original mechanical type bars might not entangle if struck in rapid sequence. “Columns” of keys run in roughly straight lines, as do the “rows” of keys. However, the fingertips are not aligned in such manners, and do not move in this way when digits of the hand are flexed or extended, or moved sideways.

                                    Many attempts have been made over the last hundred years to improve keying performance by changing the keyboard layout. These include relocating keys within the standard layout, or changing the keyboard layout altogether. The keyboard has been divided into separate sections, and sets of keys (such as numerical pads) have been added. Arrangements of adjacent keys may be changed by altering spacing, offset from each other or from reference lines. The keyboard may be divided into sections for the left and right hand, and those sections may be laterally tilted and sloped and slanted.

                                    The dynamics of the operation of push-button keys are important for the user, but are difficult to measure in operation. Thus, the force-displacement characteristics of keys are commonly described for static testing, which is not indicative of actual operation. By current practise, keys on computer keyboards have fairly little displacement (about 2 mm) and display a “snap-back” resistance, that is, a decrease in operation force at the point when actuation of the key has been achieved. Instead of separate single keys, some keyboards consist of a membrane with switches underneath which, when pressed in the correct location, generate the desired input with little or no displacement felt. The major advantage of the membrane is that dust or fluids cannot penetrate it; however, many users dislike it.

                                    There are alternatives to the “one key-one character” principle; instead, one can generate inputs by various combinatory means. One is “chording”, meaning that two or more controls are operated simultaneously to generate one character. This poses demands on the memory capabilities of the operator, but requires the use of only very few keys. Other developments utilize controls other than the binary tapped push button, replacing it by levers, toggles or special sensors (such as an instrumented glove) which respond to movements of the digits of the hand.

                                    By tradition, typing and computer entry have been made by mechanical interaction between the operator’s fingers and such devices as keyboard, mouse, track ball or light pen. Yet there are many other means to generate inputs. Voice recognition appears one promising technique, but other methods can be employed. They might utilize, for example, pointing, gestures, facial expressions, body movements, looking (directing one’s gaze), movements of the tongue, breathing or sign language to transmit information and to generate inputs to a computer. Technical development in this area is very much in flux, and as the many nontraditional input devices used for computer games indicate, acceptance of devices other than the traditional binary tap-down keyboard is entirely feasible within the near future. Discussions of current keyboard devices have been provided, for example, by Kroemer (1994b) and McIntosh (1994).

                                    Displays

                                    Displays provide information about the status of equipment. Displays may apply to the operator’s visual sense (lights, scales, counters, cathode-ray tubes, flat panel electronics, etc.), to the auditory sense (bells, horns, recorded voice messages, electronically generated sounds, etc.) or to the sense of touch (shaped controls, Braille, etc.). Labels, written instructions, warnings or symbols (“icons”) may be considered special kinds of displays.

                                    The four “cardinal rules” for displays are:

                                      1. Display only that information which is essential for adequate job performance.
                                      2. Display information only as accurately as is required for the operator’s decisions and actions.
                                      3. Present information in the most direct, simple, understandable and usable form.
                                      4. Present information in such a way that failure or malfunction of the display itself will be immediately obvious.

                                             

                                            The selection of either an auditory or visual display depends on the prevailing conditions and purposes. The objective of the display may be to provide:

                                            • historical information about the past state of the system, such as the course run by a ship
                                            • status information about the current state of the system, such as the text already input into a word processor or the current position of an airplane
                                            • predictive information, such as on the future position of a ship, given certain steering settings
                                            • instructions or commands telling the operator what to do, and possibly how to do it.

                                             

                                            A visual display is most appropriate if the environment is noisy, the operator stays in place, the message is long and complex, and especially if it deals with the spatial location of an object. An auditory display is appropriate if the workplace must be kept dark, the operator moves around, and the message is short and simple, requires immediate attention, and deals with events and time.

                                            Visual Displays

                                            There are three basic types of visual displays: (1)The check display indicates whether or not a given condition exists (for example a green light indicates normal function). (2)The qualitative display indicates the status of a changing variable or its approximate value, or its trend of change (for example, a pointer moves within a “normal” range). (3) The quantitative display shows exact information that must be ascertained (for example, to find a location on a map, to read text or to draw on a computer monitor), or it may indicate an exact numerical value that must be read by the operator (for example, a time or a temperature).

                                            Design guidelines for visual displays are:

                                            • Arrange displays so that the operator can locate and identify them easily without unnecessary searching. (This usually means that the displays should be in or near the medial plane of the operator, and below or at eye height.)
                                            • Group displays functionally or sequentially so that the operator can use them easily.
                                            • Make sure that all displays are properly illuminated or illuminant, coded and labelled according to their function.
                                            • Use lights, often coloured, to indicate the status of a system (such as ON or OFF) or to alert the operator that the system, or a subsystem, is inoperative and that special action must be taken. Common meanings of light colours are listed in figure 5. Flashing red indicates an emergency condition that requires immediate action. An emergency signal is most effective when it combines sounds with a flashing red light.

                                            Figure 5. Colour coding of indicator lights

                                            ERG210T4

                                            For more complex and detailed information, especially quantitative information, one of four different kinds of displays are traditionally used: (1) a moving pointer (with fixed scale), (2) a moving scale (with fixed pointer), (3) counters or (4) “pictorial” displays, especially computer-generated on a display monitor. Figure 6 lists the major characteristics of these display types.

                                            Figure 6. Characteristics of displays

                                            ERG210T5

                                            It is usually preferable to use a moving pointer rather than a moving scale, with the scale either straight (horizontally or vertically arranged), curved or circular. Scales should be simple and uncluttered, with graduation and numbering so designed that correct readings can be taken quickly. Numerals should be located outside the scale markings so that they are not obscured by the pointer. The pointer should end with its tip directly at the marking. The scale should mark divisions only so finely as the operator must read. All major marks should be numbered. Progressions are best marked with intervals of one, five or ten units between major marks. Numbers should increase left to right, bottom to top or clockwise. For details of dimensions of scales refer to standards such as those listed by Cushman and Rosenberg 1991 or Kroemer 1994a.

                                            Starting in the 1980s, mechanical displays with pointers and printed scales were increasingly replaced by “electronic” displays with computer-generated images, or solid-state devices using light-emitting diodes (see Snyder 1985a). The displayed information may be coded by the following means:

                                            • shapes, such as straight or circular
                                            • alphanumeric, that is, letters, numbers, words, abbreviations
                                            • figures, pictures, pictorials, icons, symbols, in various levels of abstraction, such as the outline of an airplane against the horizon
                                            • shades of black, white or gray
                                            • colours.

                                             

                                            Unfortunately, many electronically generated displays have been fuzzy, often overly complex and colourful, hard to read, and required exact focusing and close attention, which may distract from the main task, for example, driving a car. In these cases the first three of the four “cardinal rules” listed above were often violated. Furthermore, many electronically generated pointers, markings and alphanumerics did not comply with established ergonomic design guidelines, especially when generated by line segments, scan lines or dot matrices. Although some of these defective designs were tolerated by the users, rapid innovation and improving display techniques allows many better solutions. However, the same rapid development leads to the fact that printed statements (even if current and comprehensive when they appear) are becoming obsolete quickly. Therefore, none are given in this text. Compilations have been published by Cushman and Rosenberg (1991), Kinney and Huey (1990), and Woodson, Tillman and Tillman (1991).

                                            The overall quality of electronic displays is often wanting. One measure used to assess the image quality is the modulation transfer function (MTF) (Snyder 1985b). It describes the resolution of the display using a special sine-wave test signal; yet, readers have many criteria regarding the preference of displays (Dillon 1992).

                                            Monochrome displays have only one colour, usually either green, yellow, amber, orange or white (achromatic). If several colours appear on the same chromatic display, they should be easily discriminated. It is best to display not more than three or four colours simultaneously (with preference being given to red, green, yellow or orange, and cyan or purple). All should strongly contrast with the background. In fact, a suitable rule is to design first by contrast, that is, in terms of black and white, and then to add colours sparingly.

                                            In spite of the many variables that, singly and interacting with each other, affect the use of complex colour display, Cushman and Rosenberg (1991) compiled guidelines for use of colour in displays; these are listed in figure 7.

                                            Figure 7. Guidelines for use of colours in displays

                                            ERG210T6

                                            Other suggestions are as follows:

                                            • Blue (preferably desaturated) is a good colour for backgrounds and large shapes. However, blue should not be used for text, thin lines or small shapes.
                                            • The colour of alphanumeric characters should contrast with that of the background.
                                            • When using colour, use shape as a redundant cue (e.g., all yellow symbols are triangles, all green symbols are circles, all red symbols are squares). Redundant coding makes the display much more acceptable for users who have colour-vision deficiencies.
                                            • As the number of colours is increased, the sizes of the colour-coded objects should also be increased.
                                            • Red and green should not be used for small symbols and small shapes in peripheral areas of large displays.
                                            • Using opponent colours (red and green, yellow and blue) adjacent to one another or in an object/background relationship is sometimes beneficial and sometimes detrimental. No general guidelines can be given; a solution should be determined for each case.
                                            • Avoid displaying several highly saturated, spectrally extreme colours at the same time.

                                             

                                            Panels of Controls and Displays

                                            Displays as well as controls should be arranged in panels so they are in front of the operator, that is, close to the person’s medial plane. As discussed earlier, controls should be near elbow height, and displays below or at eye height, whether the operator is sitting or standing. Infrequently operated controls, or less important displays, can be located further to the sides, or higher.

                                            Often, information on the result of control operation is displayed on an instrument. In this case, the display should be located close to the control so that the control setting can be done without error, quickly and conveniently. The assignment is usually clearest when the control is directly below or to the right of the display. Care must be taken that the hand does not cover the display when operating the control.

                                            Popular expectancies of control-display relations exist, but they are often learned, they may depend on the user’s cultural background and experience, and these relationships are often not strong. Expected movement relationships are influenced by the type of control and display. When both are either linear or rotary, the stereotypical expectation is that they move in corresponding directions, such as both up or both clockwise. When the movements are incongruent, in general the following rules apply:

                                            • Clockwise for increase. Turning the control clockwise causes an increase in the displayed value.
                                            • Warrick’s gear-slide rule. A display (pointer) is expected to move in the same direction as does the side of the control close to (i.e., geared with) the display.

                                             

                                            The ratio of control and display displacement (C/D ratio or D/C gain) describes how much a control must be moved to adjust a display. If much control movement produces only a small display motion, once speaks of a high C/D ratio, and of the control as having low sensitivity. Often, two distinct movements are involved in making a setting: first a fast primary (“slewing”) motion to an approximate location, then a fine adjustment to the exact setting. In some cases, one takes as the optimal C/D ratio that which minimizes the sum of these two movements. However, the most suitable ratio depends on the given circumstances; it must be determined for each application.

                                            Labels and Warnings

                                            Labels

                                            Ideally, no label should be required on equipment or on a control to explain its use. Often, however, it is necessary to use labels so that one may locate, identify, read or manipulate controls, displays or other equipment items. Labelling must be done so that the information is provided accurately and rapidly. For this, the guidelines in table 4 apply.

                                            Table 4. Guidelines for labels

                                            Orientation

                                            A label and the information printed on it shall be oriented
                                            horizontally so that it can be read quickly and easily.
                                            (Note that this applies if the operator is used to reading
                                            horizontally, as in Western countries.)

                                            Location

                                            A label shall be placed on or very near the item that it
                                            identifies.

                                            Standardization

                                            Placement of all labels shall be consistent throughout the
                                            equipment and system.

                                            Equipment
                                            functions

                                            A label shall primarily describe the function (“what does it
                                            do”) of the labelled item.

                                            Abbreviations

                                            Common abbreviations may be used. If a new abbreviation is
                                            necessary, its meaning should be obvious to the reader.
                                            The same abbreviation shall be used for all tenses and for
                                            the singular and plural forms of a word. Capital letters
                                            shall be used, periods normally omitted.

                                            Brevity

                                            The label inscription shall be as concise as possible without
                                            distorting the intended meaning or information. The texts
                                            shall be unambiguous, redundancy minimized.

                                            Familiarity

                                            Words shall be chosen, if possible, that are familiar to the
                                            operator.

                                            Visibility and
                                            legibility

                                            The operator shall be able to be read easily and accurately at
                                            the anticipated actual reading distances, at the anticipated
                                            worst illumination level, and within the anticipated
                                            vibration and motion environment. Important factors are:
                                            contrast between the lettering and its background; the
                                            height, width, strokewidth, spacing and style of letters;
                                            and the specular reflection of the background, cover or
                                            other components.

                                            Font and size

                                            Typography determines the legibility of written information;
                                            it refers to style, font, arrangement and appearance.

                                            Source: Modified from Kroemer, Kroemer and Kroemer-Elbert 1994
                                            (reproduced by permission of Prentice-Hall; all rights reserved).

                                             

                                            Font (typeface) should be simple, bold and vertical, such as Futura, Helvetica, Namel, Tempo and Vega. Note that most electronically generated fonts (formed by LED, LCD or dot matrix) are generally inferior to printed fonts; thus, special attention must be paid to making these as legible as possible.

                                            • The height of characters depends on the viewing distance:

                                            viewing distance 35 cm, suggested height 22 mm

                                            viewing distance 70 cm, suggested height 50 mm

                                            viewing distance 1 m, suggested height 70 mm

                                            viewing distance 1.5 m, suggested height at least 1 cm.

                                            • The ratio of strokewidth to character height should be between 1:8 to 1:6 for black letters on white background, and 1:10 to 1:8 for white letters on black background.
                                            • The ratio of character width to character height should be about 3:5.
                                            • The space between letters should be at least one stroke width.
                                            • The space between words should be at least one character width.
                                            • For continuous text, mix upper- and lower-case letters; for labels, use upper-case letters only.

                                             

                                            Warnings

                                            Ideally, all devices should be safe to use. In reality, often this cannot be achieved through design. In this case, one must warn users of the dangers associated with product use and provide instructions for safe use to prevent injury or damage.

                                            It is preferable to have an “active” warning, usually consisting of a sensor that notices inappropriate use, combined with an alerting device that warns the human of an impending danger. Yet, in most cases, “passive” warnings are used, usually consisting of a label attached to the product and of instructions for safe use in the user manual. Such passive warnings rely completely on the human user to recognize an existing or potential dangerous situation, to remember the warning, and to behave prudently.

                                            Labels and signs for passive warnings must be carefully designed by following the most recent government laws and regulations, national and international standards, and the best applicable human engineering information. Warning labels and placards may contain text, graphics, and pictures—often graphics with redundant text. Graphics, particularly pictures and pictograms, can be used by persons with different cultural and language backgrounds, if these depictions are selected carefully. However, users with different ages, experiences, and ethnic and educational backgrounds, may have rather different perceptions of dangers and warnings. Therefore, design of a safe product is much preferable to applying warnings to an inferior product.

                                             

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                                            Contents

                                            Preface
                                            Part I. The Body
                                            Part II. Health Care
                                            Part III. Management & Policy
                                            Part IV. Tools and Approaches
                                            Part V. Psychosocial and Organizational Factors
                                            Part VI. General Hazards
                                            Part VII. The Environment
                                            Part VIII. Accidents and Safety Management
                                            Part IX. Chemicals
                                            Part X. Industries Based on Biological Resources
                                            Part XI. Industries Based on Natural Resources
                                            Part XII. Chemical Industries
                                            Part XIII. Manufacturing Industries
                                            Part XIV. Textile and Apparel Industries
                                            Part XV. Transport Industries
                                            Part XVI. Construction
                                            Part XVII. Services and Trade
                                            Part XVIII. Guides