EXPOSURE LIMITS

Exposure limits specify the maximal allowed exposures for individuals to chemical substances or other obnoxious influences such as noise or radiation. Such limits are usually expressed as environmental concentrations (e.g., 0.1 mg/m3 [milligrams per cubic meter] of atmosphere). Biological limits, expressed as blood concentrations, are used for some substances.

Exposure limits apply to all persons in regard to the environment, food, water, and consumer products. Public health, agricultural, and environmental protection agencies in most countries determine public exposure limits covering a wide variety of natural and nonnatural circumstances. The exposure limits that most affect us in our daily lives are probably those that limit the intake of toxic substances through food and drinking water. The area in which exposure limits have been most fully developed, however, are in relation to occupational health regulations.

Occupational Exposure Limits

Occupational exposure limits depend on specific theories about relations between exposures and harms, and on empirical data that can be brought to bear on particular cases. In some cases harms or responses do not begin until a certain threshold of exposure or dose is reached. The other argues that response is continuous from the most minimal exposure (see Figure 1).

There are also different ethical viewpoints about the degree to which workers should be protected in the workplace. One view, for instance, argues that workers are compensated for their exposure to certain possible harms by their wages and salaries, and that the only issue at most is educating them about their exposures. Another view is that workers should be no more exposed to environmental harms in the workplace than out of it. Disagreements between these two ethical views, combined with disagreements about dose–response relations, can lead to quite different interpretations of empirical data relevant to the establishment of occupational exposure limits.

The first occupational exposure limits were proposed by individual researchers in the 1880s. In the 1920s and 1930s several lists of exposure limits were published in both Europe and the United States, not always with clear identification of the dose–response relations or ethical views on which they were based. The term occupational exposure limit (OEL) was introduced in 1977 by the International Labour Organization (ILO). Other names for occupational exposure standards include threshold limit value (TLV), maximum allowed concentration (MAC), and permissible exposure limit (PEL).

Threshold Limit Values

The American Conference of Governmental Industrial Hygienists (ACGIH) was founded in 1938. In 1941 it set up the Threshold Limit Values for Chemical Substances Committee, which in 1946 issued the first list of TLVs covering around 140 chemical substances. This annually revised list has a dominant role as a standard reference for official lists all over the world.

The first TLV committee was dominated by industrial hygienists and included no physicians. Gradually, medical and scientific expertise was incorporated in the committee. In 1962 the first "Documentation of the TLVs" was published. It contained, for each substance on the list, a brief summary of its effects, with references and with grounds for the TLV that had been chosen.

In the 1940s and 1950s the ACGIH and the American Standards Association (ASA; now the American National Standards Institute [ANSI]) competed for the position of leading setter of occupational exposure limits. The exposure limits of the ASA and those of the ACGIH did not differ much in numerical terms, but the ASA values were ceiling values below which all workplace concentrations should fluctuate, whereas the ACGIH values were (and still are, with few exceptions) upper limits for the average during a whole working day. Therefore, the ASA standards were more expensive for industry but provided greater protection for exposed workers.

The ACGIH won the struggle and emerged in the early 1960s as virtually the only source of exposure limits that practitioners looked to for guidance. In 1969 the U.S. federal Occupational Safety and Health Administration (OSHA) adopted the ACGIH's exposure limits as an official standard. Because of the sluggishness of legal processes, however, OSHA has not always adopted the updated values subsequently issued by the ACGIH.

In the 1980s, the ACGIH was again challenged. The National Institute for Occupational Safety and Health (NIOSH) criticized its TLVs for being too high, and therefore not protecting workers against potential disease. The alternative values proposed by NIOSH were often many times lower than the TLVs. At the same time, OSHA was criticized for being too harsh on industry. Once again, attacks on the ACGIH were unsuccessful, and the organization retained its position as the leading setter of occupational exposure limits.

Several explanations have been given about why the ACGIH and its TLVs have been so successful. The ACGIH was first with a comprehensive listing of all-important chemicals for which measurement methods were available. As a voluntary body it has been able to update its list annually without the time- and resource-consuming legal procedures that precede revisions of OSHA standards. Furthermore, the comparative ease with which the TLVs can be implemented has probably contributed to their success. Most competing exposure limits, such as those of NIOSH, are more costly and therefore give rise to more opposition from industry.

At the same time, the TLVs have been criticized for being insufficiently protective. Examples of harmful effects at levels below the TLVs are easily found. Grace E. Ziem and Barry I. Castleman (1989) reviewed the contents of four major peer-reviewed journals in occupational medicine for thirty-three months, from January 1987 to September 1989, and found thirty-one papers that described harmful effects at or below the TLVs.

Another common criticism is that the ACGIH has relied too much on unpublished corporate information. Many values have been based on information from a company to the effect that a certain level has been found to be safe, or that no evidence of damage to health has been found at a certain level. This type of information does not satisfy modern criteria for science-based health assessment. Nevertheless, ACGIH's list of TLVs would have covered many fewer substances if such corporate information had not been used. The present policy of the ACGIH is that TLVs "represent a scientific opinion based on a review of existing peer-reviewed scientific literature" (ACGIH website).

Exposure Limits in Other Countries

Since the 1970s most industrialized countries have official lists of occupational exposure limits. In many cases, these lists developed out of the ACGIH list. Because of the less litigious legal culture in Europe, many European countries have national lists of occupational exposure limits that are updated regularly. In some countries such as Sweden and Denmark, the national list has significantly lower values than the ACGIH list.

Developing countries often use the ACGIH list with few or no modifications. As the ACGIH has itself pointed out, however, some TLVs may be unsuitable for use in countries with different conditions from those in the United States, for instance in terms of the nutritional status of workers. The ACGIH also points out that the TLVs "are not developed for use as legal standards and ACGIH does not advocate their use as such."

Some countries, such as the Netherlands and Sweden, have developed an elaborate bipartition of the regulatory task into scientific and policy components. The scientific component is performed by experts in the relevant scientific fields. It derives its legitimacy from the expertise of those who perform it. The policy component is performed by decision makers in government agencies. This component of the process can, in a democratic society, derive its legitimacy only from the same source as other political or administrative processes, meaning that those who perform it must represent the people.

Difficulties in Setting Exposure Limits

The two major sources of knowledge for setting exposure limits are epidemiological studies and animal experiments. In an epidemiological study, groups of humans are statistically compared in search of associations between disease incidence and environmental or other causal factors. The effects of major workplace hazards, such as asbestos, lead, and vinyl chloride, have been convincingly identified and quantified in epidemiological studies. At the same time many epidemiological studies are inconclusive because of the multiplicity of factors that can influence the prevalence of disease in human populations. Epidemiology also has the crucial disadvantage that the toxic effects of a substance can be discovered only when workers have already been subjected to these effects.

In animal experiments, the health status of exposed animals is compared to that of an unexposed control group. Because of the high degree of biochemical and physiological similarity between humans and the common experimental animals, animal experimentation has predictive power, but unfortunately the predictions are far from perfect. There are substances to which humans are much more, or much less, sensitive than the common laboratory animals.

Because of the uncertainty inherent in both epidemiology and animal experiments, it is in practice virtually impossible to determine with certainty absolutely safe nonzero levels of toxic exposure. Furthermore, for genotoxic carcinogens, it is generally believed that although the risk diminishes with the exposure, it is not completely eliminated until the exposure has been reduced to zero. Accordingly, the ACGIH has stated that the TLVs "represent conditions under which ACGIH believes that nearly all workers may be repeatedly exposed without adverse health effects. They are not fine lines between safe and dangerous exposures, nor are they a relative index of toxicology" (ACGIH website). Other setters of exposure limits have made similar statements.

To set occupational exposure limits is no easy task. Workers exposed to potentially dangerous substances expect exposure limits to fully protect their health. Employers expect the exposure limits to impose only such costs as are necessary to protect employee health. It is in practice impossible to set OELs that fully satisfy both demands. The task of standard setters is to find a reasonable compromise. To achieve this is a science-based enterprise in the sense of making use of scientific information, but not in the sense of being based exclusively on science. It is in fact both science-based and value-based.

As already indicated, the determination of exposure limits involves not just empirical data but also scientific theories about how this data should be interpreted and ethical views about how it should be applied. In some cases the application of very safe exposure limits can put an industrial operation out of business, so that workers are fully protected but only at the cost of losing their jobs. In other cases, not to apply strong exposure limits can have deadly consequences. The adjudication of exposure limits in the workplace, as outside the workplace, is an issues that involves scientific and ethical education on the part of workers, employers, politicians, and citizens.

SVEN OVE HANSSON

SEE ALSO Limits;Radiation; Risk-Cost-Benefit Analysis.

BIBLIOGRAPHY

Cook, Warren A. (1985). "History of ACGIH TLVs." Annals of the American Conference of Governmental Industrial Hygienists 12: 3–9.

Hansson, Sven Ove. (1998). Setting the Limit: Occupational Health Standards and the Limits of Science. New York: Oxford University Press. Comparative study of practices in Germany, Sweden, and the UK.

Paull, Jeffrey M. (1984). "The Origin and Basis of Threshold Limit Values." American Journal of Industrial Medicine 5(3): 227–238. Early history of TLVs.

Ziem, Grace E., and Barry I. Castleman. (1989). "Threshold Limit Values: Historical Perspectives and Current Practice." Journal of Occupational Medicine 31(11): 910–918. Critical appraisal of TLVs.

INTERNET RESOURCE

American Conference of Governmental Industrial Hygienists (ACGIH). 2005. "Statement of Position Regarding the TLVs and BEIs" Available from http://acgih.org/TLV/PosStmt.htm.