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Friday, 25 March 2011 05:38

Vibration

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Vibration is oscillatory motion. This chapter summarizes human responses to whole-body vibration, hand-transmitted vibration and the causes of motion sickness.

Whole-body vibration occurs when the body is supported on a surface which is vibrating (e.g., when sitting on a seat which vibrates, standing on a vibrating floor or recumbent on a vibrating surface). Whole-body vibration occurs in all forms of transport and when working near some industrial machinery.

Hand-transmitted vibration is the vibration that enters the body through the hands. It is caused by various processes in industry, agriculture, mining and construction where vibrating tools or workpieces are grasped or pushed by the hands or fingers. Exposure to hand-transmitted vibration can lead to the development of several disorders.

Motion sickness can be caused by low frequency oscillation of the body, some types of rotation of the body and movement of displays relative to the body.

Magnitude

Oscillatory displacements of an object involve alternately a velocity in one direction and then a velocity in the opposite direction. This change of velocity means that the object is constantly accelerating, first in one direction and then in the opposite direction. The magnitude of a vibration can be quantified by its displacement, its velocity or its acceleration. For practical convenience, the acceleration is usually measured with accelerometers. The units of acceleration are metres per second per second (m/s2). The acceleration due to the Earth’s gravity is approximately 9.81 m/s2.

The magnitude of an oscillation can be expressed as the distance between the extremities reached by the motion (the peak-to-peak value) or the distance from some central point to the maximum deviation (the peak value). Often, the magnitude of vibration is expressed in terms of an average measure of the acceleration of the oscillatory motion, usually the root-mean-square value (m/s2 r.m.s.). For a single frequency (sinusoidal) motion, the r.m.s. value is the peak value divided by √2.

For a sinusoidal motion the acceleration, a (in m/s2), can be calculated from the frequency, f (in cycles per second), and the displacement, d (in metres):

a=(f)2d

This expression may be used to convert acceleration measurements to displacements, but it is only accurate when the motion occurs at a single frequency.

Logarithmic scales for quantifying vibration magnitudes in decibels are sometimes used. When using the reference level in International Standard 1683, the acceleration level, La, is expressed by La = 20 log10(a/a0), where a is the measured acceleration (in m/s2 r.m.s.) and a0 is the reference level of 10-6 m/s2. Other reference levels are used in some countries.

 

Frequency

The frequency of vibration, which is expressed in cycles per second (hertz, Hz), affects the extent to which vibration is transmitted to the body (e.g., to the surface of a seat or the handle of a vibratory tool), the extent to which it is transmitted through the body (e.g., from the seat to the head), and the effect of vibration in the body. The relation between the displacement and the acceleration of a motion are also dependent on the frequency of oscillation: a displacement of one millimetre corresponds to a very low acceleration at low frequencies but a very high acceleration at high frequencies; the vibration displacement visible to the human eye does not provide a good indication of vibration acceleration.

The effects of whole-body vibration are usually greatest at the lower end of the range, from 0.5 to 100 Hz. For hand-transmitted vibration, frequencies as high as 1,000 Hz or more may have detrimental effects. Frequencies below about 0.5 Hz can cause motion sickness.

The frequency content of vibration can be shown in spectra. For many types of whole-body and hand-transmitted vibration the spectra are complex, with some motion occurring at all frequencies. Nevertheless, there are often peaks, which show the frequencies at which most of the vibration occurs.

Since human responses to vibration vary according to the vibration frequency, it is necessary to weight the measured vibration according to how much vibration occurs at each frequency. Frequency weightings reflect the extent to which vibration causes the undesired effect at each frequency. Weightings are required for each axis of vibration. Different frequency weightings are required for whole-body vibration, hand-transmitted vibration and motion sickness.

Direction

Vibration may take place in three translational directions and three rotational directions. For seated persons, the translational axes are designated x-axis (fore-and-aft), y-axis (lateral) and
z-axis (vertical). Rotations about the x-, y- and z-axes are designated rx (roll), ry (pitch) and rz (yaw), respectively. Vibration is usually measured at the interfaces between the body and the vibration. The principal coordinate systems for measuring vibration with respect to whole-body and hand-transmitted vibration are illustrated in the next two articles in the chapter.

Duration

Human responses to vibration depend on the total duration of vibration exposure. If the characteristics of vibration do not change with time, the root-mean-square vibration provides a convenient measure of the average vibration magnitude. A stopwatch may then be sufficient to assess the exposure duration. The severity of the average magnitude and total duration can be assessed by reference to the standards in the following articles.

If the vibration characteristics vary, the measured average vibration will depend on the period over which it is measured. Furthermore, root-mean-square acceleration is believed to underestimate the severity of motions which contain shocks, or are otherwise highly intermittent.

Many occupational exposures are intermittent, vary in magnitude from moment to moment or contain occasional shocks. The severity of such complex motions can be accumulated in a manner which gives appropriate weight to, for example, short periods of high magnitude vibration and long periods of low magnitude vibration. Different methods of calculating doses are used (see “Whole-body vibration”; “Hand-transmitted vibration”; and “Motion sickness” in this chapter).

 

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Friday, 25 March 2011 05:41

Whole-Body Vibration

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Occupational Exposure

Occupational exposures to whole-body vibration mainly occur in transport but also in association with some industrial processes. Land, sea and air transport can all produce vibration that can cause discomfort, interfere with activities or cause injury. Table 1 lists some environments which may be most likely to be associated with a health risk.


Table 1. Activities for which it may be appropriate to warn of the adverse effects of whole-body vibration

Tractor driving

Armoured fighting vehicles (e.g., tanks) and similar vehicles

Other off-road vehicles:

Earth-moving machinery—loaders, excavators, bulldozers, graders,

  • scrapers, dumpers, rollers
  • Forest machines
  • Mine and quarry equipment
  • Forklift trucks

 

Some truck driving (articulated and non-articulated)

Some bus and tram driving

Some helicopter and fixed-wing aircraft flying

Some workers with concrete production machinery

Some railway drivers

Some use of high-speed marine craft

Some motor bicycle riding

Some car and van driving

Some sports activities

Some other industrial equipment

Source: Adapted from Griffin 1990. 


The most common exposure to severe vibration and shocks may occur on off-road vehicles, including earth moving machinery, industrial trucks and agricultural tractors.

Biodynamics

Like all mechanical structures, the human body has resonance frequencies where the body exhibits a maximum mechanical response. Human responses to vibration cannot be explained solely in terms of a single resonance frequency. There are many resonances in the body, and the resonance frequencies vary among people and with posture. Two mechanical responses of the body are often used to describe the manner in which vibration causes the body to move: transmissibility and impedance.

The transmissibility shows the fraction of the vibration which is transmitted from, say, the seat to the head. The transmissibility of the body is highly dependent on vibration frequency, vibration axis and body posture. Vertical vibration on a seat causes vibration in several axes at the head; for vertical head motion, the transmissibility tends to be greatest in the approximate range of 3 to 10 Hz.

The mechanical impedance of the body shows the force that is required to make the body move at each frequency. Although the impedance depends on body mass, the vertical impedance of the human body usually shows a resonance at about 5 Hz. The mechanical impedance of the body, including this resonance, has a large effect on the manner in which vibration is transmitted through seats.

Acute Effects

Discomfort

The discomfort caused by vibration acceleration depends on the vibration frequency, the vibration direction, the point of contact with the body, and the duration of vibration exposure. For vertical vibration of seated persons, the vibration discomfort caused by any frequency increases in proportion to the vibration magnitude: a halving of the vibration will tend to halve the vibration discomfort.

The discomfort produced by vibration may be predicted by the use of appropriate frequency weightings (see below) and described by a semantic scale of discomfort. There are no useful limits for vibration discomfort: the acceptable discomfort varies from one environment to another.

Acceptable magnitudes of vibration in buildings are close to vibration perception thresholds. The effects on humans of vibration in buildings are assumed to depend on the use of the building in addition to the vibration frequency, direction and duration. Guidance on the evaluation of building vibration is given in various standards such as British Standard 6472 (1992) which defines a procedure for the evaluation of both vibration and shock in buildings.

Activity interference

Vibration can impair the acquisition of information (e.g., by the eyes), the output of information (e.g., by hand or foot movements) or the complex central processes that relate input to output (e.g., learning, memory, decision-making). The greatest effects of whole-body vibration are on input processes (mainly vision) and output processes (mainly continuous hand control).

Effects of vibration on vision and manual control are primarily caused by the movement of the affected part of the body (i.e., eye or hand). The effects may be decreased by reducing the transmission of vibration to the eye or to the hand, or by making the task less susceptible to disturbance (e.g., increasing the size of a display or reducing the sensitivity of a control). Often, the effects of vibration on vision and manual control can be much reduced by redesign of the task.

Simple cognitive tasks (e.g., simple reaction time) appear to be unaffected by vibration, other than by changes in arousal or motivation or by direct effects on input and output processes. This may also be true for some complex cognitive tasks. However, the sparsity and diversity of experimental studies does not exclude the possibility of real and significant cognitive effects of vibration. Vibration may influence fatigue, but there is little relevant scientific evidence, and none which supports the complex form of the “fatigue-decreased proficiency limit” offered in International Standard 2631 (ISO 1974, 1985).

Changes in Physiological Functions

Changes in physiological functions occur when subjects are exposed to a novel whole-body vibration environment in laboratory conditions. Changes typical of a “startle response” (e.g., increased heart rate) normalize quickly with continuing exposure, whereas other reactions either proceed or develop gradually. The latter can depend on all characteristics of vibration including the axis, the magnitude of acceleration, and the kind of vibration (sinusoidal or random), as well as on further variables such as circadian rhythm and characteristics of the subjects (see Hasan 1970; Seidel 1975; Dupuis and Zerlett 1986). Changes of physiological functions under field conditions often cannot be related to vibration directly, since vibration is often acting together with other significant factors, such as high mental strain, noise and toxic substances. Physiological changes are frequently less sensitive than psychological reactions (e.g., discomfort). If all available data on persistent physiological changes are summarized with respect to their first significant appearance depending on the magnitude and frequency of whole-body vibration, there is a boundary with a lower border around 0.7 m/s2 r.m.s. between 1 and 10 Hz, and rising up to 30 m/s2 r.m.s. at 100 Hz. Many animal studies have been performed, but their relevance to humans is doubtful.

Neuromuscular changes

During active natural motion, motor control mechanisms act as a feed-forward control that is constantly adjusted by additional feedback from sensors in muscles, tendons and joints. Whole-body vibration causes a passive artificial motion of the human body, a condition that is fundamentally different from the self-induced vibration caused by locomotion. The missing feed-forward control during whole-body vibration is the most distinct change of the normal physiological function of the neuromuscular system. The broader frequency range associated with whole-body vibration (between 0.5 and 100 Hz) compared to that for natural motion (between 2 and 8 Hz for voluntary movements, and below 4 Hz for locomotion) is a further difference that helps to explain reactions of the neuromuscular control mechanisms at very low and at high frequencies.

Whole-body vibration and transient acceleration cause an acceleration-related alternating activity in the electromyogram (EMG) of superficial back muscles of seated persons that requires a tonic contraction to be maintained. This activity is supposed to be of a reflex-like nature. It usually disappears completely if the vibrated subjects sit relaxed in a bent position. The timing of muscle activity depends on the frequency and magnitude of acceleration. Electromyographic data suggest that an increased spinal load can occur due to reduced muscular stabilization of the spine at frequencies from 6.5 to 8 Hz and during the initial phase of a sudden upward displacement. In spite of weak EMG activity caused by whole-body vibration, back muscle fatigue during vibration exposure can exceed that observed in normal sitting postures without whole-body vibration.

Tendon reflexes may be diminished or disappear temporarily during exposure to sinusoidal whole-body vibration at frequencies above 10 Hz. Minor changes of postural control after exposure to whole-body vibration are quite variable, and their mechanisms and practical significance are not certain.

Cardiovascular, respiratory, endocrine and metabolic changes

The observed changes persisting during exposure to vibration have been compared to those during moderate physical work (i.e., increases of heart rate, blood pressure and oxygen consumption) even at a vibration magnitude near to the limit of voluntary tolerance. The increased ventilation is partially caused by oscillations of the air in the respiratory system. Respiratory and metabolic changes may not correspond, possibly suggesting a disturbance of the respiration control mechanisms. Various and partially contradictory findings have been reported for changes of the adrenocorticotropic hormones (ACTH) and catecholamines.

Sensory and central nervous changes

Changes of vestibular function due to whole-body vibration have been claimed on the basis of an affected regulation of posture, although posture is controlled by a very complex system in which a disturbed vestibular function can be largely compensated by other mechanisms. Changes of the vestibular function seem to gain significance for exposures with very low frequencies or those near the resonance of the whole body. A sensory mismatch between vestibular, visual and proprioceptive (stimuli received within the tissues) information is supposed to be an important mechanism underlying physiological responses to some artificial motion environments.

Experiments with short-term and prolonged combined exposures to noise and whole-body vibration, seem to suggest that vibration has a minor synergistic effect on hearing. As a tendency, high intensities of whole-body vibration at 4 or 5 Hz were associated with higher additional temporary threshold shifts (TTS). There was no obvious relation between the additional TTS and exposure time. The additional TTS seemed to increase with higher doses of whole-body vibration.

Impulsive vertical and horizontal vibrations evoke brain potentials. Changes of the function of the human central nervous system have also been detected using auditory evoked brain potentials (Seidel et al. 1992). The effects were influenced by other environmental factors (e.g., noise), the difficulty of the task, and by the internal state of the subject (e.g., arousal, degree of attention towards the stimulus).

Long-Term Effects

Spinal health risk

Epidemiological studies have frequently indicated an elevated health risk for the spine in workers exposed for many years to intense whole-body vibration (e.g., work on tractors or earth-moving machines). Critical surveys of the literature have been prepared by Seidel and Heide (1986), Dupuis and Zerlett (1986) and Bongers and Boshuizen (1990). These reviews concluded that intense long-term whole-body vibration can adversely affect the spine and can increase the risk of low-back pain. The latter may be a secondary consequence of a primary degenerative change of the vertebrae and disks. The lumbar part of the vertebral column was found to be the most frequently affected region, followed by the thoracic region. A high rate of impairments of the cervical part, reported by several authors, seems to be caused by a fixed unfavourable posture rather than by vibration, although there is no conclusive evidence for this hypothesis. Only a few studies have considered the function of back muscles and found a muscular insufficiency. Some reports have indicated a significantly higher risk of the dislocation of lumbar disks. In several cross-sectional studies Bongers and Boshuizen (1990) found more low-back pain in drivers and helicopter pilots than in comparable reference workers. They concluded that professional vehicle driving and helicopter flying are important risk factors for low-back pain and back disorder. An increase in disability pensioning and long-term sick leave due to intervertebral disc disorders was observed among crane operators and tractor drivers.

Due to incomplete or missing data on exposure conditions in epidemiological studies, exact exposure-effect relationships have not been obtained. The existing data do not permit the substantiation of a no-adverse-effect level (i.e., safe limit) so as to reliably prevent diseases of the spine. Many years of exposure below or near the exposure limit of the current International Standard 2631 (ISO 1985) are not without risk. Some findings have indicated an increasing health risk with increased duration of exposure, although selection processes have made it difficult to detect a relation in the majority of studies. Thus, a dose-effect relationship cannot currently be established by epidemiological investigations. Theoretical considerations suggest marked detrimental effects of high peak loads acting on the spine during exposures with high transients. The use of an “energy equivalent” method to calculate a vibration dose (as in International Standard 2631 (ISO 1985)) is therefore questionable for exposures to whole-body vibration containing high peak accelerations. Different long-term effects of whole-body vibration depending on the vibration frequency have not been derived from epidemiological studies. Whole-body vibration at 40 to 50 Hz applied to standing workers through the feet was followed by degenerative changes of the bones of the feet.

In general, differences between subjects have been largely neglected, although selection phenomena suggest they may be of major importance. There are no clear data showing whether the effects of whole-body vibration on the spine depend on gender.

The general acceptance of degenerative disorders of the spine as an occupational disease is debated. Specific diagnostic features are not known which would permit a reliable diagnosis of the disorder as an outcome of exposure to whole-body vibration. A high prevalence of degenerative spinal disorders in non-exposed populations hinders the assumption of a predominantly occupational aetiology in individuals exposed to whole-body vibration. Individual constitutional risk factors that might modify vibration-induced strain are unknown. The use of a minimal intensity and/or a minimal duration of whole-body vibration as a prerequisite for the recognition of an occupational disease would not take into account the expected considerable variability in individual susceptibility.

Other health risks

Epidemiological studies suggest that whole-body vibration is one factor within a causative set of factors which contribute to other health risks. Noise, high mental strain and shift work are examples of important concomitant factors which are known to be associated with health disorders. The results of investigations into disorders of other bodily systems have often been divergent or have indicated a paradoxical dependence of the prevalence of pathology on the magnitude of whole-body vibration (i.e., a higher prevalence of adverse effects with a lower intensity). A characteristic complex of symptoms and pathological changes of the central nervous system, the musculo-skeletal system and the circulatory system has been observed in workers standing on machines used for the vibro-compression of concrete and exposed to whole-body vibration beyond the exposure limit of ISO 2631 with frequencies above 40 Hz (Rumjancev 1966). This complex was designated as “vibration disease”. Although rejected by many specialists, the same term has sometimes been used to describe a vague clinical picture caused by long-term exposure to low-frequency whole-body vibration which, allegedly, is manifested initially as peripheral and cerebral vegeto-vascular disorders with a non-specific functional character. Based on the available data it can be concluded that different physiological systems react independently of one another and that there are no symptoms which might serve as an indicator of pathology induced by whole-body vibration.

Nervous system, vestibular organ and hearing. Intense whole-body vibration at frequencies higher than 40 Hz can cause damage and disturbances of the central nervous system. Conflicting data have been reported on effects of whole-body vibration at frequencies below 20 Hz. In some studies only, an increase of non-specific complaints such as headache and increased irritability has been found. Disturbances of the electroencephalogram (EEG) after long-term exposure to whole-body vibration have been claimed by one author and denied by others. Some published results are consistent with a decreased vestibular excitability and a higher incidence of other vestibular disturbances, including dizziness. However, it remains doubtful whether there are causal links between whole-body vibration and changes in the central nervous system or vestibular system because paradoxical intensity-effect relationships were detected.

In some studies, an additional increase of the permanent threshold shifts (PTS) of hearing has been observed after a combined long-term exposure to whole-body vibration and noise. Schmidt (1987) studied drivers and technicians in agriculture and compared the permanent threshold shifts after 3 and 25 years on the job. He concluded that whole-body vibration can induce an additional significant threshold shift at 3, 4, 6 and 8 kHz, if the weighted acceleration according to International Standard 2631 (ISO 1985) exceeds 1.2 m/s2 r.m.s. with a simultaneous exposure to noise at an equivalent level of more than 80 decibels (dBA).

Circulatory and digestive systems. Four main groups of circulatory disturbances have been detected with a higher incidence among workers exposed to whole-body vibration:

  1. peripheral disorders, such as the Raynaud-syndrome, near to the site of application of whole-body vibration (i.e., the feet of standing workers or, with a low degree only, the hands of drivers)
  2. varicose veins of the legs, haemorrhoids and varicocele
  3. ischaemic heart disease and hypertension
  4. neurovascular changes.

 

The morbidity of these circulatory disturbances did not always correlate with the magnitude or duration of vibration exposure. Although a high prevalence of various disorders of the digestive system has often been observed, almost all authors agree that whole-body vibration is but one cause and possibly not the most important.

Female reproductive organs, pregnancy and male urogenital system. Increased risks of abortions, menstrual disturbances and anomalies of positions (e.g., uterine descent) have been assumed to be associated with long-term exposure to whole-body vibration (see Seidel and Heide 1986). A safe exposure limit in order to avoid a higher risk for these health risks cannot be derived from the literature. The individual susceptibility and its temporal changes probably co-determine these biological effects. In the available literature, a harmful direct effect of whole-body vibration on the human foetus has not been reported, although some animal studies suggest that whole-body vibration can affect the foetus. The unknown threshold value for adverse effects on pregnancy suggests a limitation on an occupational exposure to the lowest reasonable extent.

Divergent results have been published for the occurrence of diseases of the male urogenital system. In some studies, a higher incidence of prostatitis was observed. Other studies could not confirm these findings.

Standards

No precise limit can be offered to prevent disorders caused by whole-body vibration, but standards define useful methods of quantifying vibration severity. International Standard 2631 (ISO 1974, 1985) defined exposure limits (see figure 1) which were “set at approximately half the level considered to be the threshold of pain (or limit of voluntary tolerance) for healthy human subjects ”. Also shown in figure 1 is a vibration dose value action level for vertical vibration derived from British Standard 6841 (BSI 1987b); this standard is, in part, similar to a draft revision of the International Standard.

Figure 1. Frequency dependencies for human response to whole-body vibration

VIB020F1

The vibration dose value can be considered to be the magnitude of a one-second duration of vibration which will be equally severe to the measured vibration. The vibration dose value uses a fourth-power time dependency to accumulate vibration severity over the exposure period from the shortest possible shock to a full day of vibration (e.g., BSI 6841):

Vibration dose value = 

The vibration dose value procedure can be used to evaluate the severity of both vibration and repetitive shocks. This fourth-power time dependency is simpler to use than the time dependency in ISO 2631 (see figure 2).

Figure 2. Time dependencies for human response to a whole-body vibration

VIB020F2

British Standard 6841 offers the following guidance.

High vibration dose values will cause severe discomfort, pain and injury. Vibration dose values also indicate, in a general way, the severity of the vibration exposures which caused them. However there is currently no consensus of opinion on the precise relation between vibration dose values and the risk of injury. It is known that vibration magnitudes and durations which produce vibration dose values in the region of 15 m/s1.75 will usually cause severe discomfort. It is reasonable to assume that increased exposure to vibration will be accompanied by increased risk of injury (BSI 1987b).

At high vibration dose values, prior consideration of the fitness of the exposed persons and the design of adequate safety precautions may be required. The need for regular checks on the health of routinely exposed persons may also be considered.

The vibration dose value provides a measure by which highly variable and complex exposures can be compared. Organizations may specify limits or action levels using the vibration dose value. For example, in some countries, a vibration dose value of 15 m/s1.75 has been used as a tentative action level, but it may be appropriate to limit vibration or repeated shock exposures to higher or lower values depending on the situation. With current understanding, an action level merely serves to indicate the approximate values that might be excessive. Figure 2 illustrates the root-mean-square accelerations corresponding to a vibration dose value of 15 m/s1.75 for exposures between one second and 24 hours. Any exposure to continuous vibration, intermittent vibration, or repeated shock may be compared with the action level by calculating the vibration dose value. It would be unwise to exceed an appropriate action level (or the exposure limit in ISO 2631) without consideration of the possible health effects of an exposure to vibration or shock.

The Machinery Safety Directive of the European Economic Community states that machinery must be designed and constructed so that hazards resulting from vibration produced by the machinery are reduced to the lowest practicable level, taking into account technical progress and the availability of means of reducing vibration. The Machinery Safety Directive (Council of the European Communities 1989) encourages the reduction of vibration by means additional to reduction at source (e.g., good seating).

Measurement and Evaluation of Exposure

Whole-body vibration should be measured at the interfaces between the body and the source of vibration. For seated persons this involves the placement of accelerometers on the seat surface beneath the ischial tuberosities of subjects. Vibration is also sometimes measured at the seat back (between the backrest and the back) and also at the feet and hands (see figure 3).

Figure 3. Axes for measuring vibration exposures of seated persons

VIB020F3

Epidemiological data alone are not sufficient to define how to evaluate whole-body vibration so as to predict the relative risks to health from the different types of vibration exposure. A consideration of epidemiological data in combination with an understanding of biodynamic responses and subjective responses is used to provide current guidance. The manner in which the health effects of oscillatory motions depend upon the frequency, direction and duration of motion is currently assumed to be the same as, or similar to, that for vibration discomfort. However, it is assumed that the total exposure, rather than the average exposure, is important, and so a dose measure is appropriate.

In addition to evaluating the measured vibration according to current standards, it is advisable to report the frequency spectra, magnitudes in different axes and other characteristics of the exposure, including the daily and lifetime exposure durations. The presence of other adverse environmental factors, especially sitting posture, should also be considered.

 

 

 

Prevention

Wherever possible, reduction of vibration at the source is to be preferred. This may involve reducing the undulations of the terrain or reducing the speed of travel of vehicles. Other methods of reducing the transmission of vibration to operators require an understanding of the characteristics of the vibration environment and the route for the transmission of vibration to the body. For example, the magnitude of vibration often varies with location: lower magnitudes will be experienced in some areas. Table 2 lists some preventive measures that may be considered.


Table 2. Summary of preventive measures to consider when persons are exposed to whole-body vibration

Group

Action

Management

Seek technical advice

 

Seek medical advice

 

Warn exposed persons

 

Train exposed persons

 

Review exposure times

 

Have policy on removal from exposure

Machine manufacturers

Measure vibration

 

Design to minimize whole-body vibration

 

Optimize suspension design

 

Optimize seating dynamics

 

Use ergonomic design to provide good posture etc.

 

Provide guidance on machine maintenance

 

Provide guidance on seat maintenance

 

Provide warning of dangerous vibration

Technical-at workplace

Measure vibration exposure

 

Provide appropriate machines

 

Select seats with good attenuation

 

Maintain machines

 

Inform management

Medical

Pre-employment screening

 

Routine medical checks

 

Record all signs and reported symptoms

 

Warn workers with apparent predisposition

 

Advise on consequences of exposure

 

Inform management

Exposed persons

Use machine properly

 

Avoid unnecessary vibration exposure

 

Check seat is properly adjusted

 

Adopt good sitting posture

 

Check condition of machine

 

Inform supervisor of vibration problems

 

Seek medical advice if symptoms appear

 

Inform employer of relevant disorders

Source: Adapted from Griffin 1990.


Seats can be designed to attenuate vibration. Most seats exhibit a resonance at low frequencies, which results in higher magnitudes of vertical vibration occurring on the seat than on the floor! At high frequencies there is usually attenuation of vibration. In use, the resonance frequencies of common seats are in the region of 4 Hz. The amplification at resonance is partially determined by the damping in the seat. An increase in the damping of the seat cushioning tends to reduce the amplification at resonance but increase the transmissibility at high frequencies. There are large variations in transmissibility between seats, and these result in significant differences in the vibration experienced by people.

A simple numerical indication of the isolation efficiency of a seat for a specific application is provided by the seat effective amplitude transmissibility (SEAT) (see Griffin 1990). A SEAT value greater than 100% indicates that, overall, the vibration on the seat is worse than the vibration on the floor. Values below 100% indicate that the seat has provided some useful attenuation. Seats should be designed to have the lowest SEAT value compatible with other constraints.

A separate suspension mechanism is provided beneath the seat pan in suspension seats. These seats, used in some off-road vehicles, trucks and coaches, have low resonance frequencies (around 2 Hz) and so can attenuate vibration at frequencies above about 3 Hz. The transmissibilities of these seats are usually determined by the seat manufacturer, but their isolation efficiencies vary with operating conditions.

 

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Friday, 25 March 2011 05:48

Hand-Transmitted Vibration

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Occupational Exposure

Mechanical vibration arising from powered processes or tools and entering the body at the fingers or the palm of the hands is called hand-transmitted vibration. Frequent synonyms for hand-transmitted vibration are hand-arm vibration and local or segmental vibration. Powered processes and tools which expose operators’ hands to vibration are widespread in several industrial activities. Occupational exposure to hand-transmitted vibration arises from hand-held powered tools used in manufacturing (e.g., percussive metal-working tools, grinders and other rotary tools, impact wrenches), quarrying, mining and construction (e.g., rock-drills, stone-hammers, pick-hammers, vibrocompactors), agriculture and forestry (e.g., chain saws, brush saws, barking machines) and public utilities (e.g., road and concrete breakers, drill-hammers, hand-held grinders). Exposure to hand-transmitted vibration can also occur from vibrating workpieces held in the hands of the operator as in pedestal grinding, and from hand-held vibrating controls as in operating lawn mowers or in controlling vibrating road compactors. It has been reported that the number of persons exposed to hand-transmitted vibration at work exceeds 150,000 in the Netherlands, 0.5 million in Great Britain, and 1.45 million in the United States. Excessive exposure to hand-transmitted vibration can cause disorders in the blood vessels, nerves, muscles, and bones and joints of the upper limbs. It has been estimated that 1.7 to 3.6% of the workers in European countries and the United States are exposed to potentially harmful hand-transmitted vibration (ISSA International Section for Research 1989). The term hand-arm vibration (HAV) syndrome is commonly used to refer to signs and symptoms associated with exposure to hand-transmitted vibration, which include:

  • vascular disorders
  • peripheral neurological disorders
  • bone and joint disorders
  • muscle disorders
  • other disorders (whole body, central nervous system).

 

Leisure activities such as motorcycling or using domestic vibrating tools can occasionally expose the hands to vibration of high amplitude, but only long daily exposures may give rise to health problems (Griffin 1990).

The relationship between occupational exposure to hand-transmitted vibration and adverse health effects is far from simple. Table 1 lists some of the most important factors which concur to cause injuries in the upper limbs of vibration-exposed workers.


Table 1. Some factors potentially related to injurious effects during hand-transmitted vibration exposures

Vibration characteristics

  • Magnitude (r.m.s., peak, weighted/unweighted)
  • Frequency (spectra, dominant frequencies)
  • Direction (x-, y-, z- axes)

 

Tools or processes

  • Tool design (portable, fixed)
  • Tool type (percussive, rotary, rotating percussive)
  • Condition
  • Operation
  • Material being worked

 

Exposure conditions

  • Duration (daily, yearly exposures)
  • Pattern of exposure (continuous, intermittent, rest periods)
  • Cumulative exposure duration

 

Environmental conditions

  • Ambient temperature
  • Airflow
  • Humidity
  • Noise
  • Dynamic response of the finger-hand-arm system
  • Mechanical impedance
  • Vibration transmissibility
  • Absorbed energy

 

Individual characteristics

  • Method of working (grip force, push force, hand-arm posture, body position)
  • Health
  • Training
  • Skill
  • Use of gloves
  • Individual susceptibility to injury 

Biodynamics

It may be presumed that factors influencing the transmission of vibration into the finger-hand-arm system play a relevant role in the genesis of vibration injury. The transmission of vibration depends on both the physical characteristics of vibration (magnitude, frequency, direction) and the dynamic response of the hand (Griffin 1990).

Transmissibility and impedance

Experimental results indicate that the mechanical behaviour of the human upper limb is complex, as the impedance of the hand-arm system—that is, its resistance to vibrate—shows pronounced variations with the change in vibration amplitude, frequency and direction, applied forces, and orientation of the hand and arm with respect to the axis of the stimulus. Impedance is also influenced by body constitution and structural differences of the various parts of the upper limb (e.g., the mechanical impedance of the fingers is much lower than that of the palm of the hand). In general, higher vibration levels, as well as tighter hand-grips, result in greater impedance. However, the change in impedance has been found to be highly dependent on the frequency and direction of the vibration stimulus and various sources of both intra- and inter-subject variability. A resonance region for the finger-hand-arm system in the frequency range between 80 and 300 Hz has been reported in several studies.

Measurements of the transmission of vibration through the human arm have shown that lower frequency vibration (>50 Hz) is transmitted with little attenuation along the hand and forearm. The attenuation at the elbow is dependent on the arm posture, as the transmission of vibration tends to decrease with the increase of the flexion angle at the elbow joint. For higher frequencies (>50 Hz), the transmission of vibration progressively decreases with increasing frequency, and above 150 to 200 Hz most of the vibrational energy is dissipated in the tissues of the hand and fingers. From transmissibility measurements it has been inferred that in the high-frequency region vibration may be responsible for damage to the soft structures of the fingers and hands, while low-frequency vibration of high amplitude (e.g., from percussive tools) might be associated with injuries to the wrist, elbow and shoulder.

Factors influencing finger and hand dynamics

The adverse effects from vibration exposure may be assumed to be related to the energy dissipated in the upper limbs. Energy absorption is highly dependent on factors affecting the coupling of the finger-hand system to the vibration source. Variations in grip pressure, static force and posture modify the dynamic response of the finger, hand and arm, and, consequently, the amount of energy transmitted and absorbed. For instance, grip pressure has a considerable influence on energy absorption and, in general, the higher the hand grip the greater the force transmitted to the hand-arm system. Dynamic response data can provide relevant information to assess the injury potential of tool vibration and to assist in the development of anti-vibration devices such as hand-grips and gloves.

Acute Effects

Subjective discomfort

Vibration is sensed by various skin mechanoreceptors, which are located in the (epi)dermal and subcutaneous tissues of the smooth and bare (glabrous) skin of the fingers and hands. They are classified into two categories—slow and fast adapting—according to their adaptation and receptive field properties. Merkel discs and Ruffini endings are found in the slow-adapting mechanoreceptive units, which respond to static pressure and slow changes in pressure and are excited at low frequency (<16 Hz). Fast-adapting units have Meissner’s corpuscles and Pacinian corpuscles, which respond to rapid changes in stimulus and are responsible for vibratory sensation in the frequency range between 8 and 400 Hz. The subjective response to hand-transmitted vibration has been used in several studies to obtain threshold values, contours of equivalent sensation and unpleasant or tolerance limits for vibratory stimuli at different frequencies (Griffin 1990). Experimental results indicate that human sensitivity to vibration decreases with increasing frequency for both comfort and annoyance vibration levels. Vertical vibration appears to cause more discomfort than vibration in other directions. Subjective discomfort has also been found to be a function of the spectral composition of vibration and the grip force exerted on the vibrating handle.

Activity interference

Acute exposure to hand-transmitted vibration can cause a temporary increase in vibrotactile thresholds due to a depression of the excitability of the skin mechanoreceptors. The magnitude of the temporary threshold shift as well as the time for recovery is influenced by several variables, such as the characteristics of the stimulus (frequency, amplitude, duration), temperature as well as the worker’s age and previous exposure to vibration. Exposure to cold aggravates the tactile depression induced by vibration, because low temperature has a vasoconstrictive effect on digital circulation and reduces finger skin temperature. In vibration-exposed workers who often operate in a cold environment, repeated episodes of acute impairment of tactile sensitivity can lead to permanent reduction in sensory perception and loss of manipulative dexterity, which, in turn, can interfere with work activity, increasing the risk for acute injuries due to accidents.

Non-Vascular Effects

Skeletal

Vibration-induced bone and joint injuries are a controversial matter. Various authors consider that disorders of bones and joints in workers using hand-held vibrating tools are not specific in character and similar to those due to the ageing process and to heavy manual work. On the other hand, some investigators have reported that characteristic skeletal changes in the hands, the wrists and the elbows can result from prolonged exposure to hand-transmitted vibration. Early x-ray investigations had revealed a high prevalence of bone vacuoles and cysts in the hands and wrists of vibration-exposed workers, but more recent studies have shown no significant increase with respect to control groups made up of manual workers. Excess prevalence of wrist osteoarthrosis and elbow arthrosis and osteophytosis has been reported in coal miners, road construction workers and metal-working operators exposed to shocks and low frequency vibration of high amplitude from pneumatic percussive tools. On the contrary, there is little evidence for an increased prevalence of degenerative bone and joint disorders in the upper limbs of workers exposed to mid- or high-frequency vibrations arising from chain saws or grinding machines. Heavy physical effort, forceful gripping and other biomechanical factors can account for the higher occurrence of skeletal injuries found in workers operating percussive tools. Local pain, swelling, and joint stiffness and deformities may be associated with radiological findings of bone and joint degeneration. In a few countries (including France, Germany, Italy), bone and joint disorders occurring in workers using hand-held vibrating tools are considered to be an occupational disease, and the affected workers are compensated.

Neurological

Workers handling vibrating tools may experience tingling and numbness in their fingers and hands. If vibration exposure continues, these symptoms tend to worsen and can interfere with work capacity and life activities. Vibration-exposed workers may exhibit increased vibratory, thermal and tactile thresholds in clinical examinations. It has been suggested that continuous vibration exposure can not only depress the excitability of skin receptors but also induce pathological changes in the digital nerves such as perineural oedema, followed by fibrosis and nerve fibre loss. Epidemiological surveys of vibration-exposed workers show that the prevalence of peripheral neurological disorders varies from a few per cent to more than 80 per cent, and that sensory loss affects users of a wide range of tool types. It seems that vibration neuropathy develops independently of other vibration-induced disorders. A scale of the neurological component of the HAV syndrome was proposed at the Stockholm Workshop 86 (1987), consisting of three stages according to the symptoms and the results of clinical examination and objective tests (table 2).

Table 2. Sensorineural stages of the Stockholm Workshop scale for the hand-arm vibration syndrome

Stage

Signs and symptoms

0SN

Exposed to vibration but no symptoms

1SN

Intermittent numbness, with or without tingling

2SN

Intermittent or persistent numbness, reduced sensory perception

3SN

Intermittent or persistent numbness, reduced tactile discrimination and/or
manipulative dexterity

Source: Stockholm Workshop 86 1987.

Careful differential diagnosis is required to distinguish vibration neuropathy from entrapment neuropathies, such as carpal tunnel syndrome (CTS), a disorder due to compression of the median nerve as it passes through an anatomical tunnel in the wrist. CTS seems to be a common disorder in some occupational groups using vibrating tools, such as rock-drillers, platers and forestry workers. It is believed that ergonomic stressors acting on the hand and wrist (repetitive movements, forceful gripping, awkward postures), in addition to vibration, can cause CTS in workers handling vibrating tools. Electroneuromyography measuring sensory and motor nerve velocities has proven to be useful to differentiate CTS from other neurological disorders.

Muscular

Vibration-exposed workers may complain of muscle weakness and pain in the hands and arms. In some individuals muscle fatigue can cause disability. A decrease in hand-grip strength has been reported in follow-up studies of lumberjacks. Direct mechanical injury or peripheral nerve damage have been suggested as possible aetiological factors for muscle symptoms. Other work-related disorders have been reported in vibration-exposed workers, such as tendinitis and tenosynovitis in the upper limbs, and Dupuytren’s contracture, a disease of the fascial tissue of the palm of the hand. These disorders seem to be related to ergonomic stress factors arising from heavy manual work, and the association with hand-transmitted vibration is not conclusive.

Vascular Disorders

Raynaud’s phenomenon

Giovanni Loriga, an Italian physician, first reported in 1911 that stone cutters using pneumatic hammers on marble and stone blocks at some yards in Rome suffered from finger blanching attacks, resembling the digital vasospastic response to cold or emotional stress described by Maurice Raynaud in 1862. Similar observations were made by Alice Hamilton (1918) among stone cutters in the United States, and later by several other investigators. In the literature various synonyms have been used to describe vibration-induced vascular disorders: dead or white finger, Raynaud’s phenomenon of occupational origin, traumatic vasospastic disease, and, more recently, vibration-induced white finger (VWF). Clinically, VWF is characterized by episodes of white or pale fingers caused by spastic closure of the digital arteries. The attacks are usually triggered by cold and last from 5 to 30 to 40 minutes. A complete loss of tactile sensitivity may be experienced during an attack. In the recovery phase, commonly accelerated by warmth or local massage, redness may appear in the affected fingers as a result of a reactive increase of blood flow in the cutaneous vessels. In the rare advanced cases, repeated and severe digital vasospastic attacks can lead to trophic changes (ulceration or gangrene) in the skin of the fingertips. To explain cold-induced Raynaud’s phenomenon in vibration-exposed workers, some researchers invoke an exaggerated central sympathetic vasoconstrictor reflex caused by prolonged exposure to harmful vibration, while others tend to emphasize the role of vibration-induced local changes in the digital vessels (e.g., thickening of the muscular wall, endothelial damage, functional receptor changes). A grading scale for the classification of VWF has been proposed at the Stockholm Workshop 86 (1987), (table 3). A numerical system for VWF symptoms developed by Griffin and based on scores for the blanching of different phalanges is also available (Griffin 1990). Several laboratory tests are used to diagnose VWF objectively. Most of these tests are based on cold provocation and the measurement of finger skin temperature or digital blood flow and pressure before and after cooling of fingers and hands.

Table 3. The Stockholm Workshop scale for staging cold-induced Raynaud’s phenomenon in the hand-arm vibration syndrome

Stage

Grade

Symptoms

0

No attacks

1

Mild

Occasional attacks affecting only the tips of one or more fingers

2

Moderate

Occasional attacks affecting distal and middle (rarely also
proximal) phalanges of one or more fingers

3

Severe

Frequent attacks affecting all phalanges of most fingers

4

Very severe

As in stage 3, with trophic skin changes in the finger tips

Source: Stockholm Workshop 86 1987.

Epidemiological studies have pointed out that the prevalence of VWF is very wide, from less than 1 to 100 per cent. VWF has been found to be associated with the use of percussive metal-working tools, grinders and other rotary tools, percussive hammers and drills used in excavation, vibrating machinery used in the forest, and other powered tools and processes. VWF is recognized as an occupational disease in many countries. Since 1975–80 a decrease in the incidence of new cases of VWF has been reported among forestry workers in both Europe and Japan after the introduction of anti-vibration chain saws and administrative measures curtailing saw usage time. Similar findings are not yet available for tools of other types.

Other Disorders

Some studies indicate that in workers affected with VWF hearing loss is greater than that expected on the basis of ageing and noise exposure from the use of vibrating tools. It has been suggested that VWF subjects may have an additional risk of hearing impairment due to vibration-induced reflex sympathetic vasoconstriction of the blood vessels supplying the inner ear. In addition to peripheral disorders, other adverse health effects involving the endocrine and central nervous system of vibration-exposed workers have been reported by some Russian and Japanese schools of occupational medicine (Griffin 1990). The clinical picture, called “vibration disease,” includes signs and symptoms related to dysfunction of the autonomic centres of the brain (e.g., persistent fatigue, headache, irritability, sleep disturbances, impotence, electroencephalographic abnormalities). These findings should be interpreted with caution and further carefully designed epidemiological and clinical research work is needed to confirm the hypothesis of an association between disorders of the central nervous system and exposure to hand-transmitted vibration.

Standards

Several countries have adopted standards or guidelines for hand-transmitted vibration exposure. Most of them are based on the International Standard 5349 (ISO 1986). To measure hand-transmitted vibration ISO 5349 recommends the use of a frequency-weighting curve which approximates the frequency-dependent sensitivity of the hand to vibration stimuli. The frequency-weighted acceleration of vibration (ah,w) is obtained with an appropriate weighting-filter or by summation of weighted acceleration values measured in octave or one-third octave bands along an orthogonal coordinate system (xh, yh, zh), (figure 1). In ISO 5349 the daily exposure to vibration is expressed in terms of energy-equivalent frequency-weighted acceleration for a period of four hours ((ah,w)eq(4) in m/s2 r.m.s), according to the following equation:

(ah,w)eq(4)=(T/4)½(ah,w)eq(T)

where T is the daily exposure time expressed in hours and (ah,w)eq(T) is the energy-equivalent frequency-weighted acceleration for the daily exposure time T. The standard provides guidance to calculate (ah,w)eq(T) if a typical work-day is characterized by several exposures of different magnitudes and durations. Annex A to ISO 5349 (which does not form part of the standard) proposes a dose-effect relationship between (ah,w)eq(4) and VWF, which can be approximated by the equation:

C=[(ah,w)eq(4) TF/95]2 x 100

where C is the percentile of exposed workers expected to show VWF (in the range 10 to 50%), and TF is the exposure time before finger blanching among the affected workers (in the range 1 to 25 years). The dominant, single-axis component of vibration directed into the hand is used to calculate (ah,w)eq(4), which should not be in excess of 50 m/s2. According to the ISO dose-effect relationship, VWF may be expected to occur in about 10% of workers with daily vibration exposure to 3 m/s2 for ten years.

 

Figure 1. Basicentric coordinate system for the measurement of hand-transmitted vibration

 

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In order to minimize the risk of vibration-induced adverse health effects, action levels and threshold limit values (TLVs) for vibration exposure have been proposed by other committees or organizations. The American Conference of Government Industrial Hygienists (ACGIH) has published TLVs for hand-transmitted vibration measured according to the ISO frequency-weighting procedure (American Conference of Governmental Industrial Hygienists 1992), (table 4). According to ACGIH, the proposal TLVs concern vibration exposure to which “nearly all workers may be exposed repeatedly without progressing beyond Stage 1 of the Stockholm Workshop Classification System for VWF”. More recently, exposure levels for hand-transmitted vibration have been presented by the Commission of the European Communities within a proposal of a Directive for the protection of workers against the risks arising from physical agents (Council of the European Union 1994), (table 5). In the proposed Directive the quantity used for the assessment of vibration hazard is expressed in terms of an eight-hour energy-equivalent frequency-weighted acceleration, A(8)=(T/8)½ (ah,w)eq(T), by using the vector sum of weighted accelerations determined in orthogonal coordinates asum=(ax,h,w2+ay,h,w2+az,h,w2)½ on the vibrating tool handle or workpiece. The methods of measurement and assessment of vibration exposure reported in the Directive are basically derived from the British Standard (BS) 6842 (BSI 1987a). The BS standard, however, does not recommend exposure limits, but provides an informative appendix on the state of knowledge of the dose-effect relationship for hand-transmitted vibration. The estimated frequency-weighted acceleration magnitudes liable to cause VWF in 10% of workers exposed to vibration according to the BS standard are reported in table 6.

___________________________________________________________________________

Table 4.  Threshold limit values for hand-transmitted vibration

Total daily exposure  (hours)              

  Frequency-weighted r.m.s. acceleration in the dominant direction that should not be exceeded

 

g*

 4-8

 4

 0.40

 2-4 

 6

 0.61

 1-2

 8

 0.81

 1

 12

 1.22

* 1 g = 9.81 .

Source: According to the American Conference of Government Industrial Hygienists 1992.

___________________________________________________________________________

Table 5.  Proposal of the Council of the European Union for a Council Directive on physical agents: Annex II A. Hand-transmitted vibration (1994)

 Levels ()

  A(8)*   

Definitions

 Threshold

  1

The exposure value below which continuous and/or repetitive

exposure has no adverse effect on health and safety of workers

 Action

  2.5

The value above which one or more of the measures**

specified in the relevant Annexes must be undertaken

 Exposure limit value  

  5

The exposure value above which an unprotected person is

exposed to unacceptable risks. Exceeding this level is

prohibited and must be prevented through the implementation

of the provisions of the Directive***

* A(8) = 8 h energy-equivalent frequency-weighted acceleration.

** Information, training, technical measures, health surveillance.

*** Appropriate measures for the protection of health and safety.

___________________________________________________________________________

Table 6.  Frequency-weighted vibration acceleration magnitudes ( r.m.s.) which may be expected to produce finger blanching in 10% of persons exposed*

  Daily exposure (hours)    

               Life-time exposure (years)

 

 0.5      

 1         

 2        

 4        

 8        

 16     

 0.25

 256.0     

 128.0     

 64.0     

 32.0     

 16.0     

 8.0     

 0.5

 179.2

 89.6

 44.8

 22.4

 11.2

 5.6

 1

 128.0

 64.0

 32.0

 16.0

 8.0

 4.0

 2

 89.6

 44.8

 22.4

 11.2

 5.6

 2.8

 4

 64.0

 32.0

 16.0

 8.0

 4.0

 2.0

 8

 44.8

 22.4

 11.2

 5.6

 2.8

 1.4

* With short duration exposure the magnitudes are high and vascular disorders may not be the first adverse symptom to develop.

Source: According to British Standard 6842. 1987, BSI 1987a.

___________________________________________________________________________

Measurement and Evaluation of Exposure

Vibration measurements are made to provide assistance for the development of new tools, to check vibration of tools at purchase, to verify maintenance conditions, and to assess human exposure to vibration at the workplace. Vibration-measuring equipment generally consists of a transducer (usually an accelerometer), an amplifying device, filter (bandpass filter and/or frequency-weighting network), and amplitude or level indicator or recorder. Vibration measurements should be made on the tool handle or workpiece close to the surface of the hand(s) where the vibration enters the body. Careful selection of the accelerometers (e.g., type, mass, sensitivity) and appropriate methods of mounting the accelerometer on the vibrating surface are required to obtain accurate results. Vibration transmitted to the hand should be measured and reported in the appropriate directions of an orthogonal coordinate system (figure 1). The measurement should be made over a frequency range of at least 5 to 1,500 Hz, and the acceleration frequency content of vibration in one or more axes can be presented in octave bands with centre frequencies from 8 to 1,000 Hz or in one-third octave bands with centre frequencies from 6.3 to 1,250 Hz. Acceleration can also be expressed as frequency-weighted acceleration by using a weighting network which complies with the characteristics specified in ISO 5349 or BS 6842. Measurements at the workplace show that different vibration magnitudes and frequency spectra can occur on tools of the same type or when the same tool is operated in a different manner. Figure 2 reports the mean value and the range of distribution of weighted accelerations measured in the dominant axis of power-driven tools used in forestry and industry (ISSA International Section for Research 1989). In several standards hand-transmitted vibration exposure is assessed in terms of four-hour or eight-hour energy-equivalent frequency-weighted acceleration calculated by means of the equations above. The method for obtaining energy-equivalent acceleration assumes that the daily exposure time required to produce adverse health effects is inversely proportional to the square of frequency-weighted acceleration (e.g., if the vibration magnitude is halved then exposure time may be increased by a factor of four). This time dependency is considered to be reasonable for standardization purposes and is convenient for instrumentation, but it should be noted that it is not fully substantiated by epidemiological data (Griffin 1990).

Figure 2. Mean values and range of distribution of frequency-weighted r.m.s. acceleration in the dominant axis measured on the handle(s) of some power tools used in forestry and industry

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Prevention

The prevention of injuries or disorders caused by hand-transmitted vibration requires the implementation of administrative, technical and medical procedures (ISO 1986; BSI 1987a). Appropriate advice to the manufacturers and users of vibrating tools should also be given. Administrative measures should include adequate information and training to instruct the operators of vibrating machinery to adopt safe and correct work practices. Since continuous exposure to vibration is believed to increase vibration hazard, work schedules should be arranged to include rest periods. Technical measures should include the choice of tools with the lowest vibration and with appropriate ergonomic design. According to the EC Directive for the safety of machinery (Council of the European Communities 1989), the manufacturer shall make public whether the frequency-weighted acceleration of hand-transmitted vibration exceeds 2.5 m/s2, as determined by suitable test codes such as indicated in the International Standard ISO 8662/1 and its companion documents for specific tools (ISO 1988). Tool maintenance conditions should be carefully checked by periodic vibration measurements. Pre-employment medical screening and subsequent clinical examinations at regular intervals should be performed on vibration-exposed workers. The aims of medical surveillance are to inform the worker of the potential risk associated with vibration exposure, to assess health status and to diagnose vibration-induced disorders at the early stage. At the first screening examination particular attention should be paid to any condition which may be aggravated by exposure to vibration (e.g., constitutional tendency to white finger, some forms of secondary Raynaud’s phenomenon, past injuries to the upper limbs, neurological disorders). Avoidance or reduction of vibration exposure for the affected worker should be decided after considering both the severity of symptoms and the characteristics of the entire working process. The worker should be advised to wear adequate clothing to keep the entire body warm, and to avoid or minimize the smoking of tobacco and the use of some drugs which can affect peripheral circulation. Gloves may be useful to protect the fingers and hands from traumas and to keep them warm. So-called anti-vibration gloves may provide some isolation of the high frequency components of vibration arising from some tools.

 

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Friday, 25 March 2011 05:56

Motion Sickness

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Motion sickness, or kinetosis, is not a pathological condition, but is a normal response to certain motion stimuli with which the individual is unfamiliar and to which he or she is, therefore, unadapted; only those without a functioning vestibular apparatus of the inner ear are truly immune.

Motions producing sickness

There are many different types of provocative motion that induce the motion sickness syndrome. Most are associated with aids to locomotion—in particular, ships, hovercraft, aircraft, automobiles and trains; less commonly, elephants and camels. The complex accelerations generated by fairground amusements, such as swings, roundabouts (merry-go-rounds), roller-coasters and so on, can be highly provocative. In addition, many astronauts/cosmonauts suffer from motion sickness (space-motion sickness) when they first make head movements in the abnormal force environment (weightlessness) of orbital flight. The motion sickness syndrome is also produced by certain moving visual stimuli, without any physical motion of the observer; the external visual world display of fixed-base simulators (simulator sickness) or a large-screen projection of scenes taken from a moving vehicle (Cinerama or IMAX sickness) are examples.

Aetiology

The essential characteristics of stimuli that induce motion sickness is that they generate discordant information from the sensory systems that provide the brain with information about the spatial orientation and motion of the body. The principal feature of this discord is a mismatch between the signals provided, principally, by the eyes and inner ear, and those that the central nervous system “expects” to receive and to be correlated.

Several categories of mismatch can be identified. Most important is the mismatch of signals from the vestibular apparatus (labyrinth) of the inner ear, in which the semicircular canals (the specialized receptors of angular accelerations) and the otolith organs (the specialized receptors of translational accelerations) do not provide concordant information. For example, when a head movement is made in a car or aircraft which is turning, both the semicircular canals and the otoliths are stimulated in an atypical manner and provide erroneous and incompatible information, information that differs substantially from that generated by the same head movement in a stable, 1-G gravity environment. Likewise, low-frequency (below 0.5 Hz) linear accelerations, such as occur aboard ship in rough seas or in an aircraft during flight through turbulent air, also generate conflicting vestibular signals and, hence, are a potent cause of motion sickness.

The mismatch of visual and vestibular information can also be an important contributory factor. The occupant of a moving vehicle who cannot see out is more likely to suffer from motion sickness than one who has a good external visual reference. The passenger below deck or in an aircraft cabin senses motion of the vehicle by vestibular cues, but he or she receives visual information only of his or her relative movement within the vehicle. The absence of an “expected” and concordant signal in a particular sensory modality is also considered to be the essential feature of visually induced motion sickness, because the visual motion cues are not accompanied by the vestibular signals that the individual “expects” to occur when subjected to the motion indicated by the visual display.

Signs and symptoms

On exposure to provocative motion, the signs and symptoms of motion sickness develop in a definite sequence, the time scale being dependent upon the intensity of the motion stimuli and the susceptibility of the individual. There are, however, considerable differences among individuals not only in susceptibility, but also in the order in which particular signs and symptoms develop, or whether they are experienced at all. Typically, the earliest symptom is epigastric discomfort (“stomach awareness”); this is followed by nausea, pallor and sweating, and is likely to be accompanied by a feeling of bodily warmth, increased salivation and eructation (belching). These symptoms commonly develop relatively slowly, but with continuing exposure to the motion, there is a rapid deterioration in well-being, the nausea increases in severity and culminates in vomiting or retching. Vomiting may bring relief, but this is likely to be short-lived unless the motion ceases.

There are other more variable features of the motion sickness syndrome. Alteration of respiratory rhythm with sighing and yawning may be an early symptom, and hyperventilation may occur, particularly in those who are anxious about the cause or consequence of their disability. Headache, tinnitus and dizziness are reported, while in those with severe malaise, apathy and depression are not uncommon, and may be of such severity that personal safety and survival are neglected. A feeling of lethargy and somnolence may be dominant following the cessation of provocative motion, and these may be the only symptoms in situations where adaptation to unfamiliar motion takes place without malaise.

Adaptation

With continued or repeated exposure to a particular provocative motion, most individuals show a decrease in the severity of symptoms; typically after three or four days of continuous exposure (as aboard ship or in a space vehicle) they have adapted to the motion and can carry out their normal duties without disability. In terms of the “mismatch” model, this adaptation or habituation represents the establishment of a new set of “expectations” in the central nervous system. However, on return to a familiar environment, these will no longer be appropriate and symptoms of motion sickness can recur (mal de débarquement) until readaptation occurs. Individuals differ considerably in the rate at which they adapt, the way they retain adaptation and the degree to which they can generalize protective adaptation from one motion environment to another. Unfortunately, a small proportion of the population (probably about 5%) do not adapt, or adapt so slowly that they continue to experience symptoms throughout the period of exposure to provocative motion.

Incidence

The incidence of sickness in a particular motion environment is governed by a number of factors, notably:

  • the physical characteristics of the motion (its intensity, frequency and direction of action)
  • the duration of exposure
  • the intrinsic susceptibility of the individual
  • the task being performed
  • other environmental factors (e.g., odour).

 

Not surprisingly, the occurrence of sickness varies widely in different motion environments. For example: nearly all the occupants of life rafts in rough seas will vomit; 60% of student aircrew members suffer from air sickness at some time during training, which in 15%is sufficiently severe to interfere with training; in contrast, less than 0.5% of passengers in civil transport aircraft are affected, although the incidence is higher in small commuter aircraft flying at low altitude in turbulent air.

Laboratory and field studies have shown that for vertical translational oscillatory motion (appropriately called heave), oscillation at a frequency of about 0.2 Hz is the most provocative (figure 1). For a given intensity (peak acceleration) of oscillation, the incidence of sickness falls quite rapidly with an increase in frequency above 0.2 Hz; motion at 1 Hz is less than one-tenth as provocative as that at 0.2 Hz. Likewise, for motion at frequencies below 0.2 Hz, although the relationship between incidence and frequency is not well defined because of a lack of experimental data; certainly, a stable, zero frequency, 1-G environment is not provocative.

Figure 1. Motion sickness incidence as a function of wave frequency and acceleration for 2 hour exposure to vertical sinusoidal motion

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Relationships established between the incidence of symptoms of motion sickness and the frequency, magnitude and duration of heave (z-axis) motion have led to the development of simple formulae that can be used to predict incidence when the physical parameters of the motion are known. The concept, embodied in British Standard 6841 (BSI 1987b) and in ISO Draft International Standard 2631-1, is that the incidence of symptoms is proportional to the Motion Sickness Dose Value (MSDVz). The MSDVz (in m/s1.5) is defined:

MSDVz=(a2t)½

where a is the root-mean-square (r.m.s.) value of the frequency-weighted acceleration (in m/s2) determined by linear integration over the duration, t (in seconds), of exposure to the motion.

The frequency weighting to be applied to the stimulus acceleration is a filter having a centre frequency and attenuation characteristics similar to those depicted in figure 1. The weighting function is defined precisely in the standards.

The percentage of an unadapted adult population (P) who are likely to vomit is given by:

P =1/3 MSDVz

Furthermore, the MSDVz may also be used to predict the level of malaise. On a four-point scale of zero (I felt all right) to three (I felt absolutely dreadful) an “illness rating” (I) is given by:

I =0.02MSDVz

Given the large differences among individuals in their susceptibility to motion sickness, the relationship between MSDVz and the occurrence of vomiting in laboratory experiments and in sea trials (figure 2) is acceptable. It should be noted that the formulae were developed from data acquired on exposures lasting from about 20 minutes to six hours with vomiting occurring in up to 70% of individuals (mostly seated) exposed to vertical, heave, motion.

 

Figure 2. Relationship between incidence of vomiting and stimulus dose (MSDV2), calculated by the procedure described in the text. Data from laboratory experiments volving  vertical oscillation (x) and sea trials (+)

 

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Knowledge about the effectiveness of translational oscillation acting in other body axes and other than in a vertical direction is fragmentary. There is some evidence from laboratory experiments on small groups of subjects that translational oscillation in a horizontal plane is more provocative, by a factor of about two, than the same intensity and frequency of vertical oscillation for seated subjects, but is less provocative, also by a factor of two, when the subject is supine and the stimulus acts in the longitudinal (z) body axis. Application of formulae and weighting characteristics embodied in standards to the prediction of sickness incidence should, therefore, be made with caution and due concern for the constraints noted above.

The considerable variability between individuals in their response to provocative motion is an important feature of motion sickness. Differences in susceptibility can, in part, be related to constitutional factors. Infants much below the age of about two years are rarely affected, but with maturation, susceptibility increases rapidly to reach a peak between four and ten years. Thereafter, susceptibility falls progressively so that the elderly are less likely to be affected, but are not immune. In any age group, females are more sensitive than males, the incidence data suggesting a ratio of approximately 1.7:1. Certain dimensions of personality, such as neuroticism, introversion and perceptual style have also been shown to be correlated, albeit weakly, with susceptibility. Motion sickness can also be a conditioned response and a manifestation of phobic anxiety.

Preventive measures

Procedures which minimize the provocative stimulus or increase the tolerance are available. These may prevent sickness in a proportion of the population, but none, other than withdrawal from the motion environment, is 100%effective. In the design of a vehicle, attention to factors which raise the frequency and reduce the magnitude of the oscillations (see figure 1) experienced by occupants during normal operation is beneficial. The provision of head support and body restraint to minimize the unnecessary head movements is advantageous, and is further aided if the occupant can assume a reclined or supine position. Sickness is less if the occupant can be given a view of the horizon; for those deprived of an external visual reference, closing the eyes reduces visual/vestibular conflict. Involvement in a task, particularly control of the vehicle, is also helpful. These measures can be of immediate benefit, but in the longer term the development of protective adaptation is of the greatest value. This is achieved by continued and repeated exposure to the motion environment, though it can be facilitated by ground-based exercises in which provocative stimuli are generated by making head movements whilst rotating on a spin table (desensitization therapy).

There are several drugs which increase tolerance, though all have side-effects (in particular, sedation), so that they should not be taken by those in primary control of a vehicle or when optimum performance is mandatory. For short-term (less than four hours) prophylaxis, 0.3 to 0.6 mg hyoscine hydrobromide (scopolamine) is recommended; longer acting are the antihistaminics, promethazine hydrochloride (25 mg), meclozine hydrochloride (50 mg), dimenhydrinate (50 mg) and cinnarizine (30 mg). The combination of either hyoscine or promethazine with 25 mg ephedrine sulphate increases prophylactic potency with some reduction of side-effects. Prophylaxis for up to 48 hours can be achieved using a scopolamine patch, which allows the drug to be slowly absorbed through the skin at a controlled rate. Effective concentrations of the drug in the body are not achieved until six to eight hours after application of the patch, so the need for this type of therapy must be anticipated.

Treatment

Those suffering from established motion sickness with vomiting should, when practicable, be placed in a position where the motion stimulus is minimized, and be given an anti–motion sickness drug, preferably promethazine by injection. Should vomiting be prolonged and repeated, intravenous replacement of fluid and electrolytes may be necessary.

 

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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