Ergonomics
ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
Ergonomic Criteria for Occupational and Public Health Surveys M. J. KARVONEN To cite this article: M. J. KARVONEN (1979) Ergonomic Criteria for Occupational and Public Health Surveys, Ergonomics, 22:6, 641-650, DOI: 10.1080/00140137908924649 To link to this article: http://dx.doi.org/10.1080/00140137908924649
Published online: 27 Mar 2007.
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Date: 31 October 2015, At: 02:16
ERGONOMICS, 1979, VOL. 22, No.6, 641-650
Ergonomic Criteria for Occupational and Public Health Surveys By M. J. KARVONEN
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Institut fiir Arbeitsphysiologie an der Universitat Dortmund. Ardeystralle 67. 4600 Dortmund I. F.R. Germany Ergonomic criteria express how load causes strain. Epidemiological studies are needed to establish these criteria and these can be further utilized in studying the effects of work on health. For establishing criteria. the exposure to the agent under study should be expressed in quantitative terms. Health-related subjective or objective end-points are available for many types of strain. It is important to indicate the confounding variables, both in regard to the work environment and to the workers studied. In some areas. ergonomic criteria are already used for standard setting. The state of the art is reviewed for noise. vibration. muscle and tendon syndromes. heart rate and accidents. With improving occupational health, the end-points available for ergonomic criteria shift from ill-health in the direction of well-being. Further research on ergonomic criteria is required for the purposes of standard setting. A systematic collection and evaluation of ergonomic criteria should be organized as an international effort. Crurres Ergonomiques Destines aux Enquetes de Medecine du Travail et de Sante Publique, Ergonomische Kril~rien fir Berujfiche und Offentliche Gesundheissuberwacnungen,
1. Introduction Ergonomics has been given various definitions. Whatever their differences may be, they all indicate that this discipl ine is concerned with the health and well-being of the workers. Health may not be the only concern of ergonomics: its findings are also relevant to the productivity of work. In different contexts one of the two aims, the humanitarian or the economic one, is more in the foreground. As the title of my presentation promises, I shall view ergonomics primarily in its relation to health. The World Health Organization defines health not only as the absence of disease and infirmity, but as complete physical, mental, and social well-being. Well-being is included in the concept of health. Being understood in this way, health is a continuum, with the idealized image of complete well-being at one end and death at the other. Between these extremes, levels of health may be selected to indicate the condition of an individual or of a population. Ergonomics deals with man in his working environment. It thus is one of the environmental health sciences. All the member sciences of this family face similar tasks: of defining and measuring exposures to the environment, of defining and measuring human form, function and well-being, and of analysing the interactions of man and environment in order to identify the causal mechanisms concerned.
2. Definition of Ergonomic Criteria The approach of environmental health research has been amply discussed during recent years. For the term •criterion' a well defined use has been adopted.. Criteria' are exposure-effect relationships: the quantitative relation between the exposure to a harmful agent in the environment and the risk or magnitude of an undesirable effect, under specified circumstances defined by environmental and target variables (Butler 1975, Jackson and Newill 1975, Knelson 1975). As applied to ergonomics, the term 0014--0139/79/22060641 $02.00 ttl [979 Taylor and Francis Ltd. ERG.
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•harmful agent' means a particular set of variables in the work system; •target' the worker or the population of workers, and' exposure' the nature and extent of interaction between the' agent' and the' target ', The word' risk' denotes the expected frequency of effects arising from a given exposure to an agent. In simple terms, ergonomic criteria express how load causes strain (Rutenfranz 1976). This definition of criteria is rigorous. In the' natural history' of environmental health research, a cause-and-effect relationship generally first becomes suspected because ofa local and/or a temporal association. Research leading to the confirmation of causality may still be conducted by using somewhat vague quantification of the exposure and of its effect. However, when practical conclusions of the research are being drawn for' derived working levels " limits, or standards, clearly defined quantitative relationships are needed. Rigorously defined criteria have been found necessary for progress in the study and management of environmental pollution. There is no reason why they should not become equally important in ergonomics.
3. Health Surveys 3.1. Objectives of Health Surveys Such ergonomic criteria which pertain to health, must first be established in health surveys. Ergonomics needs such studies for developing its research tools. The effects or work on health are the central topic of occupational health surveys. Ergonomic criteria of any kind open up possibilities to study such effects. In epidemiological studies aimed at public health problems, wide arrays of possible risk factors are often studied simultaneously. The traditional classification of occupations is n.o longer the most effective way of revealing effects of various types of work on the incidence of conditions like hypertension or arthrosis. Work systems and situations which have been identified with the aid of ergonomic criteria as unduly taxing specific capacities of the worker at work, are likely candidates also for causes of long-term damage. 3.2.
Strategies of Health Surveys Occupational and public health surveys are epidemiological studies. They refer to a population. They indicate rates; proportions of subjects with certain characteristics in a defined population. The characteristic can be an illness but also an established or suspected risk factor increasing the likelihood of illness or accident. An epidemiological study can be planned by using several strategies. A casereference study compares' affected' persons with suitably matched reference subjects, . controls'. A cohort study investigates defined groups of people. In a cross-sectional study information is obtained on the instantaneous prevalence of the conditions studied; in a longitudinal study of the incidence, i.e. the number of new events within a defined time. A longitudinal study can be either retrospective or prospective in time. The special advantage of the case-reference strategy is that risk-effect associations may be analysed by using small numbers of subjects. This again makes it practically possible to examine each subject by using extensive-and expensive-methods, Casereference studies are therefore particularly well suited for the identification of risk factors, ergonomic or otherwise. In cohort studies the number of subjects generally must be larger, and the methods of examination hence must remain simpler. Cohort studies, however, are a necessary alternative for identifying the health problems of a population, for determining the prevalence and/or incidence of these, and finally for
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interventions. An intervention is an experiment for the critical proof of the causality of an association. A suspected risk is removed, and the effect of the change is observed on the cohort. A basic tenet of experimentation is also to include a control without the intervention, a control cohort. This good old rule is, unfortunately, not always remembered.
4. Search for Harmful Agents Ergonomics is a science of the man-work interface. Its particular concern are the points of friction on this interface. Their localization may be approached from either side: in the work system or in the worker. In aiming at ergonomic criteria, both sides of the interface must be known and the interaction expressed in quantitative terms. The person nearest to the interface is the worker. He should be observed at work and his views should be consulted. His foreman comes next: he is less directly involved, but has wider experience. The ergonomist has been looking for points of friction in different work systems and can benefit from his experience. He also has checklists to aid the mapping-out of the work system. The occupational physician may have been alerted by the complaints of his patients or by the absenteeism records. The distribution of accidents to different working posts may pinpoint the work systems to which ergonomic attention should specially be directed. Ultimately, the published experience of other ergonomists is available in the scientific literature. Having used all these resources the ergonomist still may have to do some guesswork, in order to arrive at a hypothesis. As in a detective story, there are often several suspects.
5. Exposure Once a hypothesis has been formulated about the harmful components of the work system, the exposure to these agents has to be expressed in such a way as to permit a proper analysis. The exposure has to be qualitatively described in a systematic way, and its intensity and duration have to be expressed in quantitative terms. The intensity of the exposure includes the variables of load, difficulty, accuracy, and speed (cf. Frieling 1976, Luczak 1976). The total duration of the exposure may not be the sole critical variable; the alternation of different exposures and of pauses must also be considered. Standard classification and assessment schemes for the observational description of work systems have been developed (e.g. Rohmert arid Rutenfranz 1975, Rohmert and Landau 1977, Landau 1978). They often have to be completed with additional data relating to the actual ergonomic problem. For working postures, methods of assessment are available (Sarnann 1970, OW AS 1978). In repetitive assembly or packing work, however, strain may be due to such positions and movements which easily escape notice, e.g. when a joint is brought for a very short time to an extreme angle. Only an observer with expert knowledge is able in such situations to identify the critical loads. For some types of work, the relationships between intensity, duration and the resulting strain have been established in quantitative terms in laboratory studies. This applies particularly to isometric loading of muscles at more than 15 % of their maximum strength (Rohmert 1960, Corlett and Bishop 1976).
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6. End-Points of Strain An end-point often accessible to the ergonomist is the work performance. It is primarily an end-point of load, not of strain. The work performance should be related to the capacity of the worker (cf Laurig 1976). This can be done in tasks where the energy expenditure or the cardiovascular system sets the limits on work performance. However, in many applications of ergonomics the individual performance is hard to determine. For example: in studying the intake of information, its processing and decision making, the ergonomist is often left to base his conclusions on the actual work performance alone. If performance deteriorates when work is continued, the time course and extent of the decrement provide end-points which reflect strain. A decrement may also be artificially speeded up by increasing the load, e.g. by giving the worker a standardized subsidiary task. In the continuum of health, a variety oflevels may be chosen to provide end-points of strain for ergonomic criteria. A response of the organism to work may be an early one, observable during the work or for a short time after it, such as a transient shift of hearing threshold after an exposure to noise. There are also delayed responses caused by protracted work; such effects undoubtedly were more common in the centuries past, when even the physical appearance of a worker used to reveal his occupation. In an individual, stochastic health end-points are cumulative effects of repeated microtraumas caused by the work system; in a population, the frequency of accidents similarly provides a stochastic end-point (Butler 1975). Strain can be revealed by various forms of discomfort. Psycho-physical methods have been developed, especially by Srevens (1957,1966), for a quantitative evaluation of the intensities of perception. For the perception of exertion in physical exercise, workable scales exist (Borg 1974, Arstila et al. 1976). The perceived effort increases as a power function of the physical load in isometric muscle contraction and in bicycle ergometer work; the exponent is in the range from 1·6 to 2·0 (Borg 1974). Corlett and Bishop (1976) have published a 7 point scale for assessment of the general discomfort in 12 parts of body; the scale proved linearly related to the percentage of maximum holding time in static arm work at loads of30% or more of the maximum. Annoyance scales have been used for measuring population reactions to environmental noise (cf e.g, Berglund et al. 1975). In several situations subjective scaling may prove quite a sensitive instrument, but give no valuable guidance in others. Fatigue as an end-point may tum out to be quite elusive, as the classical Hawthorne study has taught us. Studying acute physiological changes during work or soon after it are standard methods in ergonomics. In dynamic work, energy expenditure, heart rate and pulmonary ventilation give largely parallel information on the strain of the organism. However, the energy expenditure also depends on body weight in most types of work. The heart rate response again varies according to the physical fitness of the worker.· When static work becomes an important component of the muscular effort, the heart rate rises relatively more than the energy expenditure. The same applies to heart rate during work in ambient heat, where blood is directed to the cutaneous circulation for heat exchange. The heart rate is also sensitive to emotional influences, particularly at low energy expenditure levels. The use of muscles, even of small ones, can be studied with electromyography. Changes in the electroencephalogram may be useful e.g. in the study of vigilance: The electrogalvanic skin response has been used as an end-point in psychological studies. With the advance of non-invasive methods and of telemetry, possibilities for
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new physiological end-points are likely to open up. In analogy to the decrement of work performance, such changes in the physiological functions may occur during protracted work, which indicate an increase of strain. Their earlier or later appearance also provides the possibility of measuring the effect of work on the organism. Work may also cause long-term physiological changes, increasing or decreasing the various capacities. The follow-up of muscle strength and of the maximum oxygen uptake capacity has proved its value in assessing the training methods of sportsmen. Short intense physical activity during leisure-time generally has a more potent training effect than eight hours of work at a moderate intensity daily. However, in ergonomic studies training changes essentially enter as confounding factors only. For damage caused by toxic agents in work environments, early indication can often be provided by the measurement of physiological variables, such as lung function or the speed of nerve conduction. Vibration injury can analogously be assessed with the aid of several physiological variables. Proceeding towards the realm of illness, the established risk factors of various diseases are available as end-points. For example: high blood pressure is not a disease per se, but it increases the risk of cardiovascular disease. Claims have been made that work characterized by factors such as overload, underload, time pressure, conflicts, responsibility etc. is associated with increased risk of cardiovascular disease. Such an effect should-at least partly-be mediated by the established risk factors, e.g. by high blood pressure or smoking. The identification of the components or characteristics of work which may have these effects is certainly worthy of ergonomic study, but no end-points for ergonomic criteria can as yet be chosen from among the established risk factors. Work can cause loss of health through illness or accidents. Noise-induced loss of hearing is recognized as the most common occupational disease. The amount of hearing left can be accurately measured with an audiometer. Tenosynovitis, muscle insertion pain and hard, tender muscles occur in repetitive or static work, and can be identified by a physician or physiotherapist. For most other diseases, however, the connection with the ergonomic character of work is presently less clear, looser, or hardly amenable to an ergonomic analysis. Absenteeism from work is a phenomenon with a complex background, reflecting not only manifest illness but also psychological and social factors. Nevertheless, the ergonomic character of work may affect it, as has been demonstrated in assembly work (Saito et al. 1972, Weitman and Davis 1972). Reduced absenteeism has also been reported as a report of psychosocial intervention, of the reorganization of assembly work for job enrichment (Novara 1973).
7. Specifying the Circumstances: Confounding Variables 7.1. Environmental Parameters An effect of an environmental agent is always produced in an environment characterized by several parameters. This applies also to effects exerted by the work system on the worker. Among the physical factors, heat exposure taxes the capacity of the cardiovascular system, but reduces the tendency to muscle tension and tenosynovitis. Cold exposure increases the sentivity of the hand circulation to vibration injury. Another physical factor to increase the sensitivity to vibration appears to be noise.
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The chemical environment may also affect several ergonomic criteria. Two examples of practical importance: variations in the partial pressure of oxygen in the air, or the presence of carbon monoxide; both are known to affect a wide spectrum of functional capacities, all the way from dark vision to cardiovascular functions. The psychological and social environment are important. They may motivate the worker to do his best, even to overtax his capacities, or frustrate him to a point where he feels unable to work. When ergonomic criteria are applied, it is obviously important to have as complete a picture as possible of the relevant environmental factors. In regard to the physical and chemical environment this is a reasonable requirement. How far the psychosocial environment can be documented, I am unable to judge.
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7.2.
Worker Parameters Once an ergonomic criterion has been established in a population of workers, this should be characterized as to sex, age and, for some purposes, also as to stature and weight. Some additional anthropometric and physiological characteristics may also be useful. However, human beings are not endowed with constant characteristics. Many of their responses to work are influenced by the circadian rhythms. This inherent intraindividual variation is firmly anchored, and potently confounds most vigilance and fatigue studies. Another parameter even more resistant to external influence is ageing. Some individual and ethnic variation in the rate of ageing exists. The age-changes of different capacities have different time scales. The same applies to the age of maximum performance capacity, as displayed e.g. in various sports events. Long-term adaptations may also in many ways change the response of man to work. Such adaptations are learning, training, detraining, and acclimatization to seasonal changes of ambient temperature or to altitude. Adapted and non-adapted workers often differ in sensitivity of responses used for ergonomic criteria.
8. From Criteria to Standards and Norms The ergonomic criteria present results of research which has been carried out under specific circumstances. The quantitative relationships thus demonstrated between the exposure and response, i.e. between load and the strain, have no claim to universal validity. There exists, however, a very real need for universally applicable recommendations, standards and norms. Even if no proper criteria are available, normative directives have to be given to those responsible for work, workers and the working environment. Criteria reduce the guesswork needed in formulating directives, or at least make guessing more intelligent. The wider the spectrum of criteria, the firmer is the basis for judgement.
9. Examples of Criteria: The State of the Art Noise as The Harmful Agent Noise exposure can be characterized in physical terms and measured. It may annoy, disturb, interfere with speech communication, cause a transient shift of hearing threshold, some changes in physiological functions controlled by the autonomous
9. I.
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nervous system and even a permanent loss of hearing. At least four of these responses -annoyance, disturbance of communication, transient shift of hearing threshold and permanent loss of hearing-are well enough studied to provide ergonomic or environmental health criteria (Parker and von Gierke 1975, Ho and Quinot 1975, Yaniv and von Gierke 1975, Arvidson and Lindvall 1978).
Vibration as The Harmful Agent Much study has been devoted to the effects of vibrating hand tools and of whole body vibration (Altman and Dittmer 1966, Parker and von Gierke 1974, Pyykko 1976. Pyykko et al. 1978). The subjective tolerance to vibration has been employed for ergonomic criteria. Vibration-induced injury in nerve conduction also makes it possible to assess the dose-response relation in the hand vibration syndrome (Alaranta and Seppalainen 1977). For injuries produced by aperiodic whole body vibration, studies of the drivers of tractors or of timber harvesting machines give some leads. Vibration standards have been recommended in several countries on the basis of the established criteria.
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9.2.
9.3.
Muscle and Tendon Syndromes Caused by Work Protracted static work can cause pain and tenderness in the muscles concerned, but also weakness and even atrophy of more peripheral muscles innervated through a nerve exposed to pressure by the affected hard muscles. The cause, static work, can be identified by observing the work. The positions of the worker can be described and the time relations of the work can be measured exactly. The external loads handled, or resistances, can be measured and hence the total loading of the muscle groups at least approximately assessed. The effect, muscle pain and tenderness, is however not a specific effect of work. The prevalence of the condition has to be studied in a group of workers and compared with a reference group. The conclusions reached can further be tested by ergonomic intervention. The necessary steps for establishing ergonomic criteria exist here. The complexity of both the cause and of its effect, however, have evidently discouraged many from proceeding to quantitative analyses leading to ergonomic criteria. Such an effort would be facilitated if the ergonomist's team included a physiotherapist or occupational therapist: Repetitive work can cause pain, tenderness and swelling around tendons (tenosynovitis) and at the insertions of muscles on bone. Although it is possible to identify causative moments in the work and to eliminate them, the syndromes nevertheless hardly appear amenable to a quantitative analysis, since they occur rather irregularly (Jarvinen and Kuorinka 1978). Their manifestation probably depends largely on some as yet incompletely understood environmental and/or worker variables. 9.4.
Heart Rate as an Indicator of Strain Work at near-maximal heart rates disproportionately increases the time needed for recovery. Frequent and protracted peak loads are therefore evidently undesirable both for the worker and for productivity. Loading the cardiovascular system is necessary training for its several functions. A 'healthy' compromise has to be reached between the two arguments, be it at work and/or during leisure. At work, high heart rates-perhaps down to some 40 % of the range from the resting to the maximum heart rate-should probably be considered as indicating undesirable strain. At lower
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heart rates, however, there is good reason to ask whether the health and the performance capacity benefit more from moderate than from minimum activity, and more from a varying than a constant load. Cardiologists would now probably vote for moderate and variable rather than for steady and minimum heart rate as the healthy alternative. Heart rate can evidently be employed over its entire range for an ergonomic criterion, but its relevance to health appears to be reversed somewhere around 100 beats min-' ..
9.5. Ergonomic Criteria in Accident Prevention The underlying hypothesis in accident prevention is that overt accident risks in work systems sooner or later cause accidents. Many of the overt risks are of an ergonomic nature, and can be identified by a systematic examination of the work process and environment (Saari 1977). Guidelines for the survey of risks and for their assessment are available (Richter 1975). Gradings of small work places according to overt accident risks due (a) to work environment, (b) to the handling of loads or, (c) to the technical equipment, have all proved to correlate with the incidence of accidents (Pekkarinen et al. 1978). The stochastic end-point of accidents at work thus proves its value for ergonomic criteria. 10. Emphasis on Protection or Enrichment? The partner disciplines of ergonomics, labour safety and industrial medicine, carry with them a tradition of fighting grave risks of accidents and occupational diseases at work. Modern technology has dramatically changed the picture on the 'factory floor'. It now is possible to turn one's attention from the sadder end of the continuum of health in the direction' well-being'. Any curtailment of health restricts the potential alternatives open to man. Thus health becomes an important determinant of the quality of life. Ergonomics has wide possibilities, not only to eliminate risks, but also to add comfort to work-and to life in general. Proper criteria will be necessary also for such ergonomics which is concerned with the positive end of the spectrum of health. 11. Plea for a Codex of Criteria Ergonomics is a young discipline. We ergonomists have not yet seen the need to formalize it. I venture to suggest that we should follow the example of the other environmental health sciences and make a systematic effort to establish ergonomic criteria. They may need ad hoc studies, but they may also be obtained with some extra effort as byproducts in research geared to other objectives. All those who apply ergonomics need recommendations, standards and norms based on ergonomic criteria. The interests of everybody would be served if the body of knowledge for establishing recommendations could be critically assessed and made available through some standard channel. To collect a codex of criteria and to keep it up-to-date would require a concerted effort by several leading experts of ergonomics. Perhaps there are volunteers who would like to have a go in some sector of ergonomics? Could they find an existing body to become the sponsor of such an experiment in international collaboration?
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M. J. Karvonen
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Ergonomic Criteria for Occupational and Public Health Surveys M. J. KARVONEN To cite this article: M. J. KARVONEN (1979) Ergonomic Criteria for Occupational and Public Health Surveys, Ergonomics, 22:6, 641-650, DOI: 10.1080/00140137908924649 To link to this article: http://dx.doi.org/10.1080/00140137908924649
Published online: 27 Mar 2007.
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Date: 31 October 2015, At: 02:16
ERGONOMICS, 1979, VOL. 22, No.6, 641-650
Ergonomic Criteria for Occupational and Public Health Surveys By M. J. KARVONEN
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Institut fiir Arbeitsphysiologie an der Universitat Dortmund. Ardeystralle 67. 4600 Dortmund I. F.R. Germany Ergonomic criteria express how load causes strain. Epidemiological studies are needed to establish these criteria and these can be further utilized in studying the effects of work on health. For establishing criteria. the exposure to the agent under study should be expressed in quantitative terms. Health-related subjective or objective end-points are available for many types of strain. It is important to indicate the confounding variables, both in regard to the work environment and to the workers studied. In some areas. ergonomic criteria are already used for standard setting. The state of the art is reviewed for noise. vibration. muscle and tendon syndromes. heart rate and accidents. With improving occupational health, the end-points available for ergonomic criteria shift from ill-health in the direction of well-being. Further research on ergonomic criteria is required for the purposes of standard setting. A systematic collection and evaluation of ergonomic criteria should be organized as an international effort. Crurres Ergonomiques Destines aux Enquetes de Medecine du Travail et de Sante Publique, Ergonomische Kril~rien fir Berujfiche und Offentliche Gesundheissuberwacnungen,
1. Introduction Ergonomics has been given various definitions. Whatever their differences may be, they all indicate that this discipl ine is concerned with the health and well-being of the workers. Health may not be the only concern of ergonomics: its findings are also relevant to the productivity of work. In different contexts one of the two aims, the humanitarian or the economic one, is more in the foreground. As the title of my presentation promises, I shall view ergonomics primarily in its relation to health. The World Health Organization defines health not only as the absence of disease and infirmity, but as complete physical, mental, and social well-being. Well-being is included in the concept of health. Being understood in this way, health is a continuum, with the idealized image of complete well-being at one end and death at the other. Between these extremes, levels of health may be selected to indicate the condition of an individual or of a population. Ergonomics deals with man in his working environment. It thus is one of the environmental health sciences. All the member sciences of this family face similar tasks: of defining and measuring exposures to the environment, of defining and measuring human form, function and well-being, and of analysing the interactions of man and environment in order to identify the causal mechanisms concerned.
2. Definition of Ergonomic Criteria The approach of environmental health research has been amply discussed during recent years. For the term •criterion' a well defined use has been adopted.. Criteria' are exposure-effect relationships: the quantitative relation between the exposure to a harmful agent in the environment and the risk or magnitude of an undesirable effect, under specified circumstances defined by environmental and target variables (Butler 1975, Jackson and Newill 1975, Knelson 1975). As applied to ergonomics, the term 0014--0139/79/22060641 $02.00 ttl [979 Taylor and Francis Ltd. ERG.
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•harmful agent' means a particular set of variables in the work system; •target' the worker or the population of workers, and' exposure' the nature and extent of interaction between the' agent' and the' target ', The word' risk' denotes the expected frequency of effects arising from a given exposure to an agent. In simple terms, ergonomic criteria express how load causes strain (Rutenfranz 1976). This definition of criteria is rigorous. In the' natural history' of environmental health research, a cause-and-effect relationship generally first becomes suspected because ofa local and/or a temporal association. Research leading to the confirmation of causality may still be conducted by using somewhat vague quantification of the exposure and of its effect. However, when practical conclusions of the research are being drawn for' derived working levels " limits, or standards, clearly defined quantitative relationships are needed. Rigorously defined criteria have been found necessary for progress in the study and management of environmental pollution. There is no reason why they should not become equally important in ergonomics.
3. Health Surveys 3.1. Objectives of Health Surveys Such ergonomic criteria which pertain to health, must first be established in health surveys. Ergonomics needs such studies for developing its research tools. The effects or work on health are the central topic of occupational health surveys. Ergonomic criteria of any kind open up possibilities to study such effects. In epidemiological studies aimed at public health problems, wide arrays of possible risk factors are often studied simultaneously. The traditional classification of occupations is n.o longer the most effective way of revealing effects of various types of work on the incidence of conditions like hypertension or arthrosis. Work systems and situations which have been identified with the aid of ergonomic criteria as unduly taxing specific capacities of the worker at work, are likely candidates also for causes of long-term damage. 3.2.
Strategies of Health Surveys Occupational and public health surveys are epidemiological studies. They refer to a population. They indicate rates; proportions of subjects with certain characteristics in a defined population. The characteristic can be an illness but also an established or suspected risk factor increasing the likelihood of illness or accident. An epidemiological study can be planned by using several strategies. A casereference study compares' affected' persons with suitably matched reference subjects, . controls'. A cohort study investigates defined groups of people. In a cross-sectional study information is obtained on the instantaneous prevalence of the conditions studied; in a longitudinal study of the incidence, i.e. the number of new events within a defined time. A longitudinal study can be either retrospective or prospective in time. The special advantage of the case-reference strategy is that risk-effect associations may be analysed by using small numbers of subjects. This again makes it practically possible to examine each subject by using extensive-and expensive-methods, Casereference studies are therefore particularly well suited for the identification of risk factors, ergonomic or otherwise. In cohort studies the number of subjects generally must be larger, and the methods of examination hence must remain simpler. Cohort studies, however, are a necessary alternative for identifying the health problems of a population, for determining the prevalence and/or incidence of these, and finally for
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interventions. An intervention is an experiment for the critical proof of the causality of an association. A suspected risk is removed, and the effect of the change is observed on the cohort. A basic tenet of experimentation is also to include a control without the intervention, a control cohort. This good old rule is, unfortunately, not always remembered.
4. Search for Harmful Agents Ergonomics is a science of the man-work interface. Its particular concern are the points of friction on this interface. Their localization may be approached from either side: in the work system or in the worker. In aiming at ergonomic criteria, both sides of the interface must be known and the interaction expressed in quantitative terms. The person nearest to the interface is the worker. He should be observed at work and his views should be consulted. His foreman comes next: he is less directly involved, but has wider experience. The ergonomist has been looking for points of friction in different work systems and can benefit from his experience. He also has checklists to aid the mapping-out of the work system. The occupational physician may have been alerted by the complaints of his patients or by the absenteeism records. The distribution of accidents to different working posts may pinpoint the work systems to which ergonomic attention should specially be directed. Ultimately, the published experience of other ergonomists is available in the scientific literature. Having used all these resources the ergonomist still may have to do some guesswork, in order to arrive at a hypothesis. As in a detective story, there are often several suspects.
5. Exposure Once a hypothesis has been formulated about the harmful components of the work system, the exposure to these agents has to be expressed in such a way as to permit a proper analysis. The exposure has to be qualitatively described in a systematic way, and its intensity and duration have to be expressed in quantitative terms. The intensity of the exposure includes the variables of load, difficulty, accuracy, and speed (cf. Frieling 1976, Luczak 1976). The total duration of the exposure may not be the sole critical variable; the alternation of different exposures and of pauses must also be considered. Standard classification and assessment schemes for the observational description of work systems have been developed (e.g. Rohmert arid Rutenfranz 1975, Rohmert and Landau 1977, Landau 1978). They often have to be completed with additional data relating to the actual ergonomic problem. For working postures, methods of assessment are available (Sarnann 1970, OW AS 1978). In repetitive assembly or packing work, however, strain may be due to such positions and movements which easily escape notice, e.g. when a joint is brought for a very short time to an extreme angle. Only an observer with expert knowledge is able in such situations to identify the critical loads. For some types of work, the relationships between intensity, duration and the resulting strain have been established in quantitative terms in laboratory studies. This applies particularly to isometric loading of muscles at more than 15 % of their maximum strength (Rohmert 1960, Corlett and Bishop 1976).
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6. End-Points of Strain An end-point often accessible to the ergonomist is the work performance. It is primarily an end-point of load, not of strain. The work performance should be related to the capacity of the worker (cf Laurig 1976). This can be done in tasks where the energy expenditure or the cardiovascular system sets the limits on work performance. However, in many applications of ergonomics the individual performance is hard to determine. For example: in studying the intake of information, its processing and decision making, the ergonomist is often left to base his conclusions on the actual work performance alone. If performance deteriorates when work is continued, the time course and extent of the decrement provide end-points which reflect strain. A decrement may also be artificially speeded up by increasing the load, e.g. by giving the worker a standardized subsidiary task. In the continuum of health, a variety oflevels may be chosen to provide end-points of strain for ergonomic criteria. A response of the organism to work may be an early one, observable during the work or for a short time after it, such as a transient shift of hearing threshold after an exposure to noise. There are also delayed responses caused by protracted work; such effects undoubtedly were more common in the centuries past, when even the physical appearance of a worker used to reveal his occupation. In an individual, stochastic health end-points are cumulative effects of repeated microtraumas caused by the work system; in a population, the frequency of accidents similarly provides a stochastic end-point (Butler 1975). Strain can be revealed by various forms of discomfort. Psycho-physical methods have been developed, especially by Srevens (1957,1966), for a quantitative evaluation of the intensities of perception. For the perception of exertion in physical exercise, workable scales exist (Borg 1974, Arstila et al. 1976). The perceived effort increases as a power function of the physical load in isometric muscle contraction and in bicycle ergometer work; the exponent is in the range from 1·6 to 2·0 (Borg 1974). Corlett and Bishop (1976) have published a 7 point scale for assessment of the general discomfort in 12 parts of body; the scale proved linearly related to the percentage of maximum holding time in static arm work at loads of30% or more of the maximum. Annoyance scales have been used for measuring population reactions to environmental noise (cf e.g, Berglund et al. 1975). In several situations subjective scaling may prove quite a sensitive instrument, but give no valuable guidance in others. Fatigue as an end-point may tum out to be quite elusive, as the classical Hawthorne study has taught us. Studying acute physiological changes during work or soon after it are standard methods in ergonomics. In dynamic work, energy expenditure, heart rate and pulmonary ventilation give largely parallel information on the strain of the organism. However, the energy expenditure also depends on body weight in most types of work. The heart rate response again varies according to the physical fitness of the worker.· When static work becomes an important component of the muscular effort, the heart rate rises relatively more than the energy expenditure. The same applies to heart rate during work in ambient heat, where blood is directed to the cutaneous circulation for heat exchange. The heart rate is also sensitive to emotional influences, particularly at low energy expenditure levels. The use of muscles, even of small ones, can be studied with electromyography. Changes in the electroencephalogram may be useful e.g. in the study of vigilance: The electrogalvanic skin response has been used as an end-point in psychological studies. With the advance of non-invasive methods and of telemetry, possibilities for
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new physiological end-points are likely to open up. In analogy to the decrement of work performance, such changes in the physiological functions may occur during protracted work, which indicate an increase of strain. Their earlier or later appearance also provides the possibility of measuring the effect of work on the organism. Work may also cause long-term physiological changes, increasing or decreasing the various capacities. The follow-up of muscle strength and of the maximum oxygen uptake capacity has proved its value in assessing the training methods of sportsmen. Short intense physical activity during leisure-time generally has a more potent training effect than eight hours of work at a moderate intensity daily. However, in ergonomic studies training changes essentially enter as confounding factors only. For damage caused by toxic agents in work environments, early indication can often be provided by the measurement of physiological variables, such as lung function or the speed of nerve conduction. Vibration injury can analogously be assessed with the aid of several physiological variables. Proceeding towards the realm of illness, the established risk factors of various diseases are available as end-points. For example: high blood pressure is not a disease per se, but it increases the risk of cardiovascular disease. Claims have been made that work characterized by factors such as overload, underload, time pressure, conflicts, responsibility etc. is associated with increased risk of cardiovascular disease. Such an effect should-at least partly-be mediated by the established risk factors, e.g. by high blood pressure or smoking. The identification of the components or characteristics of work which may have these effects is certainly worthy of ergonomic study, but no end-points for ergonomic criteria can as yet be chosen from among the established risk factors. Work can cause loss of health through illness or accidents. Noise-induced loss of hearing is recognized as the most common occupational disease. The amount of hearing left can be accurately measured with an audiometer. Tenosynovitis, muscle insertion pain and hard, tender muscles occur in repetitive or static work, and can be identified by a physician or physiotherapist. For most other diseases, however, the connection with the ergonomic character of work is presently less clear, looser, or hardly amenable to an ergonomic analysis. Absenteeism from work is a phenomenon with a complex background, reflecting not only manifest illness but also psychological and social factors. Nevertheless, the ergonomic character of work may affect it, as has been demonstrated in assembly work (Saito et al. 1972, Weitman and Davis 1972). Reduced absenteeism has also been reported as a report of psychosocial intervention, of the reorganization of assembly work for job enrichment (Novara 1973).
7. Specifying the Circumstances: Confounding Variables 7.1. Environmental Parameters An effect of an environmental agent is always produced in an environment characterized by several parameters. This applies also to effects exerted by the work system on the worker. Among the physical factors, heat exposure taxes the capacity of the cardiovascular system, but reduces the tendency to muscle tension and tenosynovitis. Cold exposure increases the sentivity of the hand circulation to vibration injury. Another physical factor to increase the sensitivity to vibration appears to be noise.
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The chemical environment may also affect several ergonomic criteria. Two examples of practical importance: variations in the partial pressure of oxygen in the air, or the presence of carbon monoxide; both are known to affect a wide spectrum of functional capacities, all the way from dark vision to cardiovascular functions. The psychological and social environment are important. They may motivate the worker to do his best, even to overtax his capacities, or frustrate him to a point where he feels unable to work. When ergonomic criteria are applied, it is obviously important to have as complete a picture as possible of the relevant environmental factors. In regard to the physical and chemical environment this is a reasonable requirement. How far the psychosocial environment can be documented, I am unable to judge.
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7.2.
Worker Parameters Once an ergonomic criterion has been established in a population of workers, this should be characterized as to sex, age and, for some purposes, also as to stature and weight. Some additional anthropometric and physiological characteristics may also be useful. However, human beings are not endowed with constant characteristics. Many of their responses to work are influenced by the circadian rhythms. This inherent intraindividual variation is firmly anchored, and potently confounds most vigilance and fatigue studies. Another parameter even more resistant to external influence is ageing. Some individual and ethnic variation in the rate of ageing exists. The age-changes of different capacities have different time scales. The same applies to the age of maximum performance capacity, as displayed e.g. in various sports events. Long-term adaptations may also in many ways change the response of man to work. Such adaptations are learning, training, detraining, and acclimatization to seasonal changes of ambient temperature or to altitude. Adapted and non-adapted workers often differ in sensitivity of responses used for ergonomic criteria.
8. From Criteria to Standards and Norms The ergonomic criteria present results of research which has been carried out under specific circumstances. The quantitative relationships thus demonstrated between the exposure and response, i.e. between load and the strain, have no claim to universal validity. There exists, however, a very real need for universally applicable recommendations, standards and norms. Even if no proper criteria are available, normative directives have to be given to those responsible for work, workers and the working environment. Criteria reduce the guesswork needed in formulating directives, or at least make guessing more intelligent. The wider the spectrum of criteria, the firmer is the basis for judgement.
9. Examples of Criteria: The State of the Art Noise as The Harmful Agent Noise exposure can be characterized in physical terms and measured. It may annoy, disturb, interfere with speech communication, cause a transient shift of hearing threshold, some changes in physiological functions controlled by the autonomous
9. I.
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nervous system and even a permanent loss of hearing. At least four of these responses -annoyance, disturbance of communication, transient shift of hearing threshold and permanent loss of hearing-are well enough studied to provide ergonomic or environmental health criteria (Parker and von Gierke 1975, Ho and Quinot 1975, Yaniv and von Gierke 1975, Arvidson and Lindvall 1978).
Vibration as The Harmful Agent Much study has been devoted to the effects of vibrating hand tools and of whole body vibration (Altman and Dittmer 1966, Parker and von Gierke 1974, Pyykko 1976. Pyykko et al. 1978). The subjective tolerance to vibration has been employed for ergonomic criteria. Vibration-induced injury in nerve conduction also makes it possible to assess the dose-response relation in the hand vibration syndrome (Alaranta and Seppalainen 1977). For injuries produced by aperiodic whole body vibration, studies of the drivers of tractors or of timber harvesting machines give some leads. Vibration standards have been recommended in several countries on the basis of the established criteria.
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9.2.
9.3.
Muscle and Tendon Syndromes Caused by Work Protracted static work can cause pain and tenderness in the muscles concerned, but also weakness and even atrophy of more peripheral muscles innervated through a nerve exposed to pressure by the affected hard muscles. The cause, static work, can be identified by observing the work. The positions of the worker can be described and the time relations of the work can be measured exactly. The external loads handled, or resistances, can be measured and hence the total loading of the muscle groups at least approximately assessed. The effect, muscle pain and tenderness, is however not a specific effect of work. The prevalence of the condition has to be studied in a group of workers and compared with a reference group. The conclusions reached can further be tested by ergonomic intervention. The necessary steps for establishing ergonomic criteria exist here. The complexity of both the cause and of its effect, however, have evidently discouraged many from proceeding to quantitative analyses leading to ergonomic criteria. Such an effort would be facilitated if the ergonomist's team included a physiotherapist or occupational therapist: Repetitive work can cause pain, tenderness and swelling around tendons (tenosynovitis) and at the insertions of muscles on bone. Although it is possible to identify causative moments in the work and to eliminate them, the syndromes nevertheless hardly appear amenable to a quantitative analysis, since they occur rather irregularly (Jarvinen and Kuorinka 1978). Their manifestation probably depends largely on some as yet incompletely understood environmental and/or worker variables. 9.4.
Heart Rate as an Indicator of Strain Work at near-maximal heart rates disproportionately increases the time needed for recovery. Frequent and protracted peak loads are therefore evidently undesirable both for the worker and for productivity. Loading the cardiovascular system is necessary training for its several functions. A 'healthy' compromise has to be reached between the two arguments, be it at work and/or during leisure. At work, high heart rates-perhaps down to some 40 % of the range from the resting to the maximum heart rate-should probably be considered as indicating undesirable strain. At lower
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heart rates, however, there is good reason to ask whether the health and the performance capacity benefit more from moderate than from minimum activity, and more from a varying than a constant load. Cardiologists would now probably vote for moderate and variable rather than for steady and minimum heart rate as the healthy alternative. Heart rate can evidently be employed over its entire range for an ergonomic criterion, but its relevance to health appears to be reversed somewhere around 100 beats min-' ..
9.5. Ergonomic Criteria in Accident Prevention The underlying hypothesis in accident prevention is that overt accident risks in work systems sooner or later cause accidents. Many of the overt risks are of an ergonomic nature, and can be identified by a systematic examination of the work process and environment (Saari 1977). Guidelines for the survey of risks and for their assessment are available (Richter 1975). Gradings of small work places according to overt accident risks due (a) to work environment, (b) to the handling of loads or, (c) to the technical equipment, have all proved to correlate with the incidence of accidents (Pekkarinen et al. 1978). The stochastic end-point of accidents at work thus proves its value for ergonomic criteria. 10. Emphasis on Protection or Enrichment? The partner disciplines of ergonomics, labour safety and industrial medicine, carry with them a tradition of fighting grave risks of accidents and occupational diseases at work. Modern technology has dramatically changed the picture on the 'factory floor'. It now is possible to turn one's attention from the sadder end of the continuum of health in the direction' well-being'. Any curtailment of health restricts the potential alternatives open to man. Thus health becomes an important determinant of the quality of life. Ergonomics has wide possibilities, not only to eliminate risks, but also to add comfort to work-and to life in general. Proper criteria will be necessary also for such ergonomics which is concerned with the positive end of the spectrum of health. 11. Plea for a Codex of Criteria Ergonomics is a young discipline. We ergonomists have not yet seen the need to formalize it. I venture to suggest that we should follow the example of the other environmental health sciences and make a systematic effort to establish ergonomic criteria. They may need ad hoc studies, but they may also be obtained with some extra effort as byproducts in research geared to other objectives. All those who apply ergonomics need recommendations, standards and norms based on ergonomic criteria. The interests of everybody would be served if the body of knowledge for establishing recommendations could be critically assessed and made available through some standard channel. To collect a codex of criteria and to keep it up-to-date would require a concerted effort by several leading experts of ergonomics. Perhaps there are volunteers who would like to have a go in some sector of ergonomics? Could they find an existing body to become the sponsor of such an experiment in international collaboration?
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