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Wednesday, 16 February 2011 00:42

Control of Indoor Environments: General Principles

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People in urban settings spend between 80 and 90% of their time in indoor spaces while carrying out sedentary activities, both during work and during leisure time. (See figure 1).

Figure 1. Urban dwellers spend 80 to 90% of their time indoors

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This fact led to the creation within these indoor spaces of environments that were more comfortable and homogeneous than those found outdoors with their changing climatic conditions. To make this possible, the air within these spaces had to be conditioned, being warmed during the cold season and cooled during the hot season.

For air conditioning to be efficient and cost-effective it was necessary to control the air coming into the buildings from the outside, which could not be expected to have the desired thermal characteristics. The result was increasingly airtight buildings and more stringent control of the amount of ambient air that was used to renew stagnant indoor air.

The energy crisis at the beginning of the 1970s—and the resulting need to save energy—represented another state of affairs often responsible for drastic reductions in the volume of ambient air used for renewal and ventilation. What was commonly done then was to recycle the air inside a building many times over. This was done, of course, with the aim of reducing the cost of air-conditioning. But something else began to happen: the number of complaints, discomfort and/or health problems of the occupants of these buildings increased considerably. This, in turn, increased the social and financial costs due to absenteeism and led specialists to study the origin of complaints that, until then, were thought to be independent of pollution.

It is not a complicated matter to explain what led to the appearance of complaints: buildings are built more and more hermetically, the volume of air supplied for ventilation is reduced, more materials and products are used to insulate buildings thermally, the number of chemical products and synthetic materials used multiplies and diversifies and individual control of the environment is gradually lost. The result is an indoor environment that is increasingly contaminated.

The occupants of buildings with degraded environments then react, for the most part, by expressing complaints about aspects of their environment and by presenting clinical symptoms. The symptoms most commonly heard of are the following sort: irritation of mucous membranes (eyes, nose and throat), headaches, shortness of breath, higher incidence of colds, allergies and so on.

When the time comes to define the possible causes that trigger these complaints, the apparent simplicity of the task gives way in fact to a very complex situation as one attempts to establish the relation of cause and effect. In this case one must look at all the factors (whether environmental or of other origins) that may be implicated in the complaints or the health problems that have appeared.

The conclusion—after many years of studying this problem—is that these problems have multiple origins. The exceptions are those cases where the relationship of cause and effect has been clearly established, as in the case of the outbreak of Legionnaires’ disease, for example, or the problems of irritation or of increased sensitivity due to exposure to formaldehyde.

The phenomenon is given the name of sick building syndrome, and is defined as those symptoms affecting the occupants of a building where complaints due to malaise are more frequent than might be reasonably expected.

Table 1 shows some examples of pollutants and the most common sources of emissions that can be associated with a drop in the quality of indoor air.

In addition to indoor air quality, which is affected by chemical and biological pollutants, sick building syndrome is attributed to many other factors. Some are physical, such as heat, noise and illumination; some are psychosocial, chief among them the way work is organized, labour relations, the pace of work and the workload.

Table 1. The most common indoor pollutants and their sources

Site

Sources of emission

Pollutant

Outdoors

Fixed sources

 
 

Industrial sites, energy production

Sulphur dioxide, nitrogen oxides, ozone, particulate matter, carbon monoxide, organic compounds

 

Motor vehicles

Carbon monoxide, lead, nitrogen oxides

 

Soil

Radon, microorganisms

Indoors

Construction materials

 
 

Stone, concrete

Radon

 

Wood composites, veneer

Formaldehyde, organic compounds

 

Insulation

Formaldehyde, fiberglass

 

Fire retardants

Asbestos

 

Paint

Organic compounds, lead

 

Equipment and installations

 
 

Heating systems, kitchens

Carbon monoxide and dioxide, nitrogen oxides, organic compounds, particulate matter

 

Photocopiers

Ozone

 

Ventilation systems

Fibres, microorganisms

 

Occupants

 
 

Metabolic activity

Carbon dioxide, water vapour, odours

 

Biological activity

Microorganisms

 

Human activity

 
 

Smoking

Carbon monoxide, other compounds, particulate matter

 

Air fresheners

Fluorocarbons, odours

 

Cleaning

Organic compounds, odours

 

Leisure, artistic activities

Organic compounds, odours

 

Indoor air plays a very important role in sick building syndrome, and controlling its quality can therefore help, in most cases, to rectify or help improve conditions that lead to the appearance of the syndrome. It should be remembered, however, that air quality is not the only factor that should be considered in evaluating indoor environments.

Measures for the Control of Indoor Environments

Experience shows that most of the problems that occur in indoor environments are the result of decisions made during the design and construction of a building. Although these problems can be solved later by taking corrective measures, it should be pointed out that preventing and correcting deficiencies during the design of the building is more effective and cost-efficient.

The great variety of possible sources of pollution determines the multiplicity of corrective actions that can be taken to bring them under control. The design of a building may involve professionals from various fields, such as architects, engineers, interior designers and others. It is therefore important at this stage to keep in mind the different factors that can contribute to eliminate or minimize the possible future problems that may arise because of poor air quality. The factors that should be considered are

  • selection of the site
  • architectural design
  • selection of materials
  • ventilation and air conditioning systems used to control the quality of indoor air.

 

Selecting a building site

Air pollution may originate at sources that are close to or far from the chosen site. This type of pollution includes, for the most part, organic and inorganic gases that result from combustion—whether from motor vehicles, industrial plants, or electrical plants near the site—and airborne particulate matter of various origins.

Pollution found in the soil includes gaseous compounds from buried organic matter and radon. These contaminants can penetrate into the building through cracks in the building materials that are in contact with the soil or by migration through semi-permeable materials.

When the construction of a building is in the planning stages, the different possible sites should be evaluated. The best site should be chosen, taking these facts and information into consideration:

  1. Data that show the levels of environmental pollution in the area, to avoid distant sources of pollution.
  2. Analysis of adjacent or nearby sources of pollution, taking into account such factors as the amount of vehicular traffic and possible sources of industrial, commercial or agricultural pollution.
  3. The levels of pollution in soil and water, including volatile or semivolatile organic compounds, radon gas and other radioactive compounds that result from the disintegration of radon. This information is useful if a decision must be made to change the site or to take measures to mitigate the presence of these contaminants within the future building. Among the measures that can be taken are the effective sealing of the channels of penetration or the design of general ventilation systems that will insure a positive pressure within the future building.
  4. Information on the climate and predominant wind direction in the area, as well as daily and seasonal variations. These conditions are important in order to decide the proper orientation of the building.

 

On the other hand, local sources of pollution must be controlled using various specific techniques, such as draining or cleaning the soil, depressurizing the soil or using architectural or scenic baffles.

Architectural design

The integrity of a building has been, for centuries, a fundamental injunction at the time of planning and designing a new building. To this end consideration has been given, today as in the past, to the capacities of materials to withstand degradation by humidity, temperature changes, air movement, radiation, the attack of chemical and biological agents or natural disasters.

The fact that the above-mentioned factors should be considered when undertaking any architectural project is not an issue in the current context: in addition, the project must implement the right decisions with regard to the integrity and well-being of the occupants. During this phase of the project decisions must be made about such concerns as the design of interior spaces, the selection of materials, the location of activities that could be potential sources of pollution, the openings of the building to the outside, the windows and the ventilation system.

Building openings

Effective measures of control during the design of the building consist of planning the location and orientation of these openings with an eye to minimizing the amount of contamination that can enter the building from previously detected sources of pollution. The following considerations should be kept in mind:

  • Openings should be far from sources of pollution and not in the predominant direction of the wind. When openings are close to sources of smoke or exhaust, the ventilation system should be planned to produce positive air pressure in that area in order to avoid the re-entry of vented air, as shown in figure 2.
  • Special attention should be given to guarantee drainage and to prevent seepage where the building comes in contact with the soil, into the foundation, in areas that are tiled, where the drainage system and conduits are located, and other sites.
  • Access to loading docks and garages should be built far from the normal air intake sites of the building as well as from the main entrances.

 

Figure 2. Penetration of pollution from the outside

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Windows

During recent years there has been a reversal of the trend seen in the 1970s and the 1980s, and now there is a tendency to include working windows in new architectural projects. This confers several advantages. One of them is the ability to provide supplementary ventilation in those areas (few in number, it is hoped) that need it, assuming that the ventilation system has sensors in those areas to prevent imbalances. It should be kept in mind that the ability to open a window does not always guarantee that fresh air will enter a building; if the ventilation system is pressurized, opening a window will not provide extra ventilation. Other advantages are of a definitely psychosocial character, allowing the occupants a certain degree of individual control over their surroundings and direct and visual access to the outdoors.

Protection against humidity

The principal means of control consist of reducing humidity in the foundations of the building, where micro-organisms, especially fungi, can frequently spread and develop.

Dehumidifying the area and pressurizing the soil can prevent the appearance of biological agents and can also prevent the penetration of chemical pollutants that may be present in the soil.

Sealing and controlling the enclosed areas of the building most susceptible to humidity in the air is another measure that should be considered, since humidity can damage the materials used to clad the building, with the result that these materials may then become a source of microbiological contamination.

Planning of indoor spaces

It is important to know during the planning stages the use to which the building will be put or the activities that will be carried out within it. It is important above all to know which activities may be a source of contamination; this knowledge can then be used to limit and control these potential sources of pollution. Some examples of activities that may be sources of contamination within a building are the preparation of food, printing and graphic arts, smoking and the use of photocopying machines.

The location of these activities in specific locales, separate and insulated from other activities, should be decided in such a way that occupants of the building are affected as little as possible.

It is advisable that these processes be provided with a localized extraction system and/or general ventilation systems with special characteristics. The first of these measures is intended to control contaminants at the source of emission. The second, applicable when there are numerous sources, when they are dispersed within a given space, or when the pollutant is extremely dangerous, should comply with the following requirements: it should be capable of providing volumes of new air which are adequate given the established standards for the activity in question, it should not reuse any of the air by mixing it with the general flow of ventilation in the building and it should include supplementary forced-air extraction where needed. In such cases the flow of air in these locales should be carefully planned, to avoid transferring pollutants between contiguous spaces—by creating, for example, negative pressure in a given space.

Sometimes control is achieved by eliminating or reducing the presence of pollutants in the air by filtration or by cleaning the air chemically. In using these control techniques, the physical and chemical characteristics of the pollutants should be kept in mind. Filtration systems, for instance, are adequate for the removal of particulate matter from the air—so long as the efficiency of the filter is matched to the size of the particles that are being filtered—but allow gases and vapours to pass through.

The elimination of the source of pollution is the most effective way to control pollution in indoor spaces. A good example that illustrates the point are the restrictions and prohibitions against smoking in the workplace. Where smoking is permitted, it is generally restricted to special areas that are equipped with special ventilation systems.

Selection of materials

In trying to prevent possible pollution problems within a building, attention should be given to the characteristics of the materials used for construction and decoration, to the furnishings, the normal work activities that will be performed, the way the building will be cleaned and disinfected and the way insects and other pests will be controlled. It is also possible to reduce the levels of volatile organic compounds (VOCs), for example, by considering only materials and furniture that have known rates of emission for these compounds and selecting those with the lowest levels.

Today, even though some laboratories and institutions have carried out studies on emissions of this kind, the information available on the rates of emission of contaminants for construction materials is scarce; this scarcity is moreover aggravated by the vast number of products available and the variability they exhibit over time.

In spite of this difficulty, some producers have begun to study their products and to include, usually at the request of the consumer or the construction professional, information on the research that has been done. Products are more and more frequently labelled environmentally safe, non-toxic and so on.

There are still many problems to overcome, however. Examples of these problems include the high cost of the necessary analyses both in time and money; the lack of standards for the methods used to assay the samples; the complicated interpretation of results obtained due to lack of knowledge of the health effects of some contaminants; and the lack of agreement among researchers on whether materials with high levels of emission that emit for a short period of time are preferable to materials with low levels of emission that emit over longer periods of time.

But the fact is that in coming years the market for construction and decoration materials will become more competitive and will come under more legislative pressure. This will result in the elimination of some products or their substitution with other products that have lower rates of emission. Measures of this sort are already being taken with the adhesives used in the production of moquette fabric for upholstery and are further exemplified by the elimination of dangerous compounds such as mercury and pentachlorophenol in the production of paint.

Until more is known and legislative regulation in this field matures, decisions as to the selection of the most appropriate materials and products to use or install in new buildings will be left to the professionals. Outlined here are some considerations that can help them arrive at a decision:

  • Information should be available on the chemical composition of the product and the emission rates of any pollutants, as well as any information regarding the health, safety and comfort of occupants exposed to them. This information should be provided by the manufacturer of the product.
  • Products should be selected which have the lowest rates of emission possible of any contaminants, giving special attention to the presence of carcinogenic and teratogenic compounds, irritants, systemic toxins, odoriferous compounds and so on. Adhesives or materials that present large emission or absorption surfaces, such as porous materials, textiles, uncoated fibres and the like, should be specified and their use restricted.
  • Preventive procedures should be implemented for the handling and installation of these materials and products. During and after the installation of these materials the space should be exhaustively ventilated and the bake out process (see below) should be used to cure certain products. The recommended hygienic measures should also be applied.
  • One of the procedures recommended to minimize exposure to emissions of new materials during the installation and finishing stages, as well as during the initial occupation of the building, is to ventilate the building for 24 hours with 100 per cent outside air. The elimination of organic compounds by the use of this technique prevents the retention of these compounds in porous materials. These porous materials may act as reservoirs and later sources of pollution as they release the stored compounds into the environment.
  • Incrementing ventilation to the maximum possible level before reoccupying a building after it has been closed for a period—during the first hours of the day—and after weekends or vacation shut-downs is also a convenient measure that can be implemented.
  • A special procedure, known as bake out, has been used in some buildings to “cure” new materials. The bake out procedure consists in elevating the temperature of a building for 48 hours or more, keeping air flow at a minimum. The high temperatures favour the emission of volatile organic compounds. The building is then ventilated and its pollution load is thereby reduced. The results obtained so far show that this procedure can be effective in some situations.

 

Ventilation systems and the control of indoor climates

In enclosed spaces, ventilation is one of the most important methods for the control of air quality. There are so many sources of pollution in these spaces, and the characteristics of these pollutants are so varied, that it is almost impossible to manage them completely in the design stage. The pollution generated by the very occupants of the building—by the activities they engage in and the products they use for personal hygiene—are a case in point; in general, these sources of contamination are beyond the control of the designer.

Ventilation is, therefore, the method of control normally used to dilute and eliminate contaminants from polluted indoor spaces; it may be carried out with clean outdoor air or recycled air that is conveniently purified.

Many different points need to be considered in designing a ventilation system if it is to serve as an adequate pollution control method. Among them are the quality of outside air that will be used; the special requirements of certain pollutants or of their generating source; the preventive maintenance of the ventilation system itself, which should also be considered a possible source of contamination; and the distribution of air inside the building.

Table 2 summarizes the main points that should be considered in the design of a ventilation system for the maintenance of quality indoor environments.

In a typical ventilation/air conditioning system, air that has been taken from outside and that has been mixed with a variable portion of recycled air passes through different air conditioning systems, is usually filtered, is heated or cooled according to the season and is humidified or dehumidified as needed.

Table 2. Basic requirements for a ventilation system by dilution

System component
or function

Requirement

Dilution by outside air

A minimum volume of air by occupant per hour should be guaranteed.

 

The aim should be to renew the volume of inside air a minimum number of times per hour.

 

The volume of outside air supplied should be increased based on the intensity of the sources of pollution.

 

Direct extraction to the outside should be guaranteed for spaces where pollution-generating activities will take place.

Air intake locations

Placing air intakes near plumes of known sources of pollution should be avoided.

 

One should avoid areas near stagnant water and the aerosols that emanate from refrigeration towers.

 

The entry of any animals should be prevented and birds should be prevented from perching or nesting near intakes.

Location of air extraction
vents

Extraction vents should be placed as far as possible from air intake locations and the height of the discharge vent should be increased.

 

Orientation of discharge vents should be in the opposite direction from air intake hoods.

Filtration and cleaning

Mechanical and electrical filters for particulate matter should be used.

 

One should install a system for the chemical elimination of pollutants.

Microbiological control

Placing any porous materials in direct contact with air currents, including those in the distribution conduits, should be avoided.

 

One should avoid the collection of stagnant water where condensation is formed in air-conditioning units.

 

A preventive maintenance programme should be established and the periodic cleaning of humidifiers and refrigeration towers should be scheduled.

Air distribution

One should eliminate and prevent the formation of any dead zones (where there is no ventilation) and the stratification of air.

 

It is preferable to mix the air where the occupants breathe it.

 

Adequate pressures should be maintained in all locales based on the activities that are performed in them.

 

Air propulsion and extraction systems should be controlled to maintain equilibrium between them.

 

Once treated, air is distributed by conduits to every area of the building and is delivered through dispersion gratings. It then mixes throughout the occupied spaces exchanging heat and renewing the indoor atmosphere before it is at last drawn away from each locale by return ducts.

The amount of outside air that should be used to dilute and to eliminate pollutants is the subject of much study and controversy. In recent years there have been changes in the recommended levels of outside air and in the published ventilation standards, in most cases involving increases in the volumes of outside air used. In spite of this, it has been noted that these recommendations are insufficient to control effectively all the sources of pollution. This is because the established standards are based on occupancy and disregard other important sources of pollution, such as the materials employed in construction, the furnishings and the quality of the air taken from the outside.

Therefore, the amount of ventilation required should be based on three fundamental considerations: the quality of air that you wish to obtain, the quality of outside air available and the total load of pollution in the space that will be ventilated. This is the starting point of the studies that have been carried out by professor PO Fanger and his team (Fanger 1988, 1989). These studies are geared to establishing new ventilation standards that meet air quality requirements and that provide an acceptable level of comfort as perceived by the occupants.

One of the factors that affects the quality of air in inside spaces is the quality of outside air available. The characteristics of exterior sources of pollution, like vehicular traffic and industrial or agricultural activities, put their control beyond the reach of the designers, the owners and the occupants of the building. It is in cases of this sort that the environmental authorities must assume the responsibility for establishing environmental protection guidelines and of making sure that they are adhered to. There are, however, many control measures that can be applied and that are useful in the reduction and the elimination of airborne pollution.

As was mentioned above, special care should be given to the location and orientation of air intake and exhaust ducts, in order to avoid drawing pollution back in from the building itself or from its installations (refrigeration towers, kitchen and bathroom vents, etc.), as well as from buildings in the immediate vicinity.

When outside air or recycled air is found to be polluted, the recommended control measures consist of filtering it and cleaning it. The most effective method of removing particulate matter is with electrostatic precipitators and mechanical retention filters. The latter will be most effective the more precisely they are calibrated to the size of the particles to be eliminated.

The use of systems capable of eliminating gases and vapours through chemical absorption and/or adsorption is a technique rarely used in nonindustrial situations; however, it is common to find systems that mask the pollution problem, especially smells for example, by the use of air fresheners.

Other techniques to clean and improve the quality of air consist of using ionizers and ozonizers. Prudence would be the best policy on the use of these systems to achieve improvements in air quality until their real properties and their possible negative health effects are clearly known.

Once air has been treated and cooled or heated it is delivered to indoor spaces. Whether the distribution of air is acceptable or not will depend, in great measure, on the selection, the number and the placement of diffusion grates.

Given the differences of opinion on the effectiveness of the different procedures that should be followed to mix air, some designers have begun to use, in some situations, air distribution systems that deliver air at floor level or on the walls as an alternative to diffusion grates on the ceiling. In any case, the location of the return registers should be carefully planned to avoid short-circuiting the entry and exit of air, which would prevent it from mixing completely as shown in figure 3.

Figure 3. Example of how air distribution can be shortcircuited in indoor spaces

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Depending on how compartmentalized workspaces are, air distribution may present a variety of different problems. For example, in open workspaces where diffusion grates are on the ceiling, air in the room may not mix completely. This problem tends to be compounded when the type of ventilation system used can supply variable volumes of air. The distribution conduits of these systems are equipped with terminals that modify the amount of air supplied to the conduits based on the data received from area thermostats.

A difficulty can develop when air flows at a reduced rate through a significant number of these terminals—a situation that arises when the thermostats of different areas reach the desired temperature—and the power to the fans that push the air is automatically reduced. The result is that the total flow of air through the system is less, in some cases much less, or even that the immission of new outside air is interrupted altogether. Placing sensors that control the flow of outside air at the intake of the system can insure that a minimum flow of new air is maintained at all times.

Another problem that regularly emerges is that air flow is blocked due to the placement of partial or total partitions in the workspace. There are many ways to correct this situation. One way is to leave an open space at the lower end of the panels that divide the cubicles. Other ways include the installation of supplementary fans and the placement of the diffusion grilles on the floor. The use of supplementary induction fan coils aid in mixing the air and allow individualized control of the thermal conditions of the given space. Without detracting from the importance of air quality per se and the means to control it, it should be kept in mind that a comfortable indoor environment is attained by the equilibrium of the different elements that affect it. Taking any action—even positive action—affecting one of the elements without regard to the rest may affect the equilibrium among them, leading to new complaints from the occupants of the building. Tables 3 and 4 show how some of these actions, intended to improve the quality of indoor air, lead to the failure of other elements in the equation, so that adjusting the working environment may have repercussions on the quality of indoor air.

Table 3. Indoor air quality control measures and their effects on indoor environments

Action

Effect

Thermal environment

Increase in volume of fresh air

Increase in draughts

Reduction of relative humidity to check microbiological agents

Insufficient relative humidity

Acoustic environment

Intermittent supplying of outside air to conserve
energy

Intermittent noise exposure

Visual environment

Reduction in the use of fluorescent lights to reduce
photochemical contamination

Reduction in the effectiveness of the illumination

Psychosocial environment

Open offices

Loss of intimacy and of a defined workspace

 

Table 4. Adjustments of the working environment and their effects on indoor air quality

Action

Effect

Thermal environment

Basing the supply of outside air on thermal
considerations

Insufficient volumes of fresh air

The use of humidifiers

Potential microbiological hazard

Acoustic environment

Increase in the use of insulating materials

Possible release of pollutants

Visual environment

Systems based solely on artificial illumination

Dissatisfaction, plant mortality, growth of microbiological agents

Psychosocial environment

Using equipment in the workspace, such as photocopiers and printer

Increase in the level of pollution

 

Insuring the quality of the overall environment of a building when it is in the design stages depends, to a great extent, on its management, but above all on a positive attitude towards the occupants of that building. The occupants are the best sensors the owners of the building can rely on in order to gauge the proper functioning of the installations intended to provide a quality indoor environment.

Control systems based on a “Big Brother” approach, making all the decisions regulating interior environments such as lighting, temperature, ventilation, and so on, tend to have a negative effect on the psychological and sociological well-being of the occupants. Occupants then see their capacity to create environmental conditions that meet their needs diminished or blocked. In addition, control systems of this type are sometimes incapable of changing to meet the different environmental requirements that may arise due to changes in the activities performed in a given space, the number of people working in it or changes in the way space is allocated.

The solution could consist of installing a system of centralized control for the indoor environment, with localized controls regulated by the occupants. This idea, very commonly used in the realm of the visual environment where general illumination is supplemented by more localized illumination, should be expanded to other concerns: general and localized heating and air-conditioning, general and localized supplies of fresh air and so on.

To sum up, it can be said that in each instance a portion of the environmental conditions should be optimized by means of a centralized control based on safety, health and economic considerations, while the different local environmental conditions should be optimized by the users of the space. Different users will have different needs and will react differently to given conditions. A compromise of this sort between the different parts will doubtless lead to greater satisfaction, well-being and productivity.

 

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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
Barometric Pressure Increased
Barometric Pressure Reduced
Biological Hazards
Disasters, Natural and Technological
Electricity
Fire
Heat and Cold
Hours of Work
Indoor Air Quality
Indoor Environmental Control
Resources
Lighting
Noise
Radiation: Ionizing
Radiation: Non-Ionizing
Vibration
Violence
Visual Display Units
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

Indoor Environmental Control Additional Resources

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Indoor Environmental Control References

American Conference of Governmental Industrial Hygienists (ACGIH). 1992. Industrial Ventilation—A Manual of Recommended Practice. 21st ed. Cincinnati, Ohio: ACGIH.

American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). 1992. Method of Testing Air Cleaner Devices Used in General Ventilation for Removing Particulate Matter. Atlanta: ASHRAE.

Baturin, VV. 1972. Fundamentals of Industrial Ventilation. New York: Pergamon.

Bedford, T and FA Chrenko. 1974. Basic Principles of Ventilation and Heating. London: HK Lewis.

Centre européen de normalisation (CEN). 1979. Method of Testing Air Filters Used in General Ventilation. Eurovent 4/5. Antwerp: European Committee of Standards.

Chartered Institution of Building Services. 1978. Environmental Criteria for Design. : Chartered Institution of Building Services.

Council of the European Communities (CEC). 1992. Guidelines for Ventilation Requirements in Buildings. Luxembourg: EC.

Constance, JD. 1983. Controlling In-Plant Airborne Contaminants. System Design and Calculations. New York: Marcel Dekker.

Fanger, PO. 1988. Introduction of the olf and the decipol units to quantify air pollution perceived by humans indoors and outdoors. Energy Build 12:7-19.

—. 1989. The new comfort equation for indoor air quality. ASHRAE Journal 10:33-38.

International Labour Organization (ILO). 1983. Encyclopaedia of Occupational Health and Safety, edited by L Parmeggiani. 3rd ed. Geneva: ILO.

National Institute for Occupational Safety and Health (NIOSH). 1991. Building Air Quality: A Guide for Building Owners and Facility Managers. Cincinnati, Ohio: NIOSH.

Sandberg, M. 1981. What is ventilation efficiency? Build Environ 16:123-135.

World Health Organization (WHO). 1987. Air Quality Guidelines for Europe. European Series, No. 23. Copenhagen: WHO Regional Publications.