|Module 1: Occupational Hygiene - Section 3: OH Standards|
|OH2.2: Development of Occupational Hygiene Standards|
The concept of occupational exposure limits originates from the shape of the exposure-response relationship. In toxicology studies, the exposure-response relationship provides data on the relationship between quantitative exposure to a hazardous agent and the effect(s) of that exposure. This relationship forms the basis of any occupational hygiene standard.
|The shape of this relationship could be straight line or a curve. In the case of a straight line (e.g. for exposure to a carcinogen like asbestos and lung cancer as an outcome) there is no safe level. For some exposures (e.g. asbestos om relation to asbestosis) there may be a lower cutoff for exposure, below which the exposure does not cause asbestosis. The exposure-response curve can therefore provide extremely important information regarding the lowest exposure at which an adverse effect can be expected. The figure on the left shows a threshold limit value, which is an exposure level below which no adverse health effect can be observed.|
Establishment of hygiene standards is based on highly technical and scientific information. Data bases used include physical and chemical properties of the substance; historical experience with the hazard; laboratory studies; and epidemiological studies. Each of these data sources has advantages and disadvantages.
Human exposure data:
Human industrial experience derives from reported cases of workplace incidents. Case studies of incidents are of limited value because of imprecise quantitative measures of actual exposures, relatively small numbers of workers involved, and inadequate health information about the persons involved.
Epidemiological studies focus on humans and may provide very useful data based on observation of typical exposures in relation to health outcomes. There is a whole science of exposure assessment and modelling which attempts to construct valid exposure estimates for workers in various occupations in different industries over time. Most epidemiological studies in occupational health are observational with little opportunity to randomly allocate exposures as in experimental studies. However, human volunteer experimental studies are possible for some substances where toxicity is low and effects are acute or immediate, facilitating study in laboratory conditions.
Animal exposure data:
Laboratory studies can also provide useful data. Here experimental studies of long term effects and chronic exposures are possible as ethical constraints are different. However, many of these studies are wasteful of life and do not really provide answers that are easily generalisable to human scenarios. The exposure-response relationships based on laboratory animal studies are subject to ongoing scientific debate regarding their relevance to human health. This is especially so in light of the large exposures delivered, and the frequency and durations of such exposure. Animals are also physiologically different from humans.
Acute and chronic toxicology studies may then be useful for evaluating short- and long-term effects on health, but extrapolating results of animal studies to humans must be done with care.
Physical and chemical properties:
Clues as to the effect of exposures derive from physical properties which indicate, for example, whether the material is likely to be present in the air under normal handling, or whether it is more likely to be in a non-volatile liquid or solid state. Similarly, the molecular weight and chemical structure may provide important clues to likely health effects. For example, some glass fibres are too large to be lodged in the deep lung and cannot therefore give rise to fibrosis. Some solvents are similar in structure to benzene which is known to cause leukemia (eg toluene or xylene). Caution is therefore indicated. Low molecular weight substances are known to combine with body proteins to cause asthma (eg detergents) and similar compounds may cause similar problems. and
The second stage in the occupational hygiene standard setting process is consideration of socio-political factors. The practical considerations relate to availability of technical expertise, costs associated with adherence and results of risk assessment. This process is usually conducted in fora where government, industry and trade unions are represented.
The negotiating process results in administrative standards. One of the expected characteristics of administrative standards is that different countries set different values for the same substance.
In South Africa the Occupational Health and Safety Act requires that "reasonably practicable" be the guiding principle when assessing standards. This implies taking into consideration the following factors: