Module 1: Occupational Hygiene - Section 3: Evaluation of Airborne Contaminants

OH3.6: Interpretation of Results - Understanding OH Standards - Resources

INTERPRETATION OF RESULTS:

Air sampling results are compared to occupational exposure limits to determine whether the plant complies with legal standards, and hence to assess the level of risk to workers.

If air monitoring results indicate that airborne concentration levels exceed applicable exposure standards, then the employer is obliged to bring down the exposure levels to the lowest level reasonably practicable using the hierarchy of controls stipulated in Regulation 10.

It is worth remembering that a sample by definition is a very small part of the larger exposure reality. It is not safe to assume that results from air sampling represent exactly the conditions of exposure all the time.

UNDERSTANDING HYGIENE STANDARDS:

Various criteria are used to evaluate employee exposure. The standards used internationally are the Threshold Limit Values (TLV) generated by the American Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH is a non-governmental organisation based in the US and its standards tend to be lower than government standards. A TLV represents conditions under which it is believed that nearly all workers may be repeatedly exposed without adverse health effects. Accordingly, ACGIH values are intended for use as guidelines or recommendations for control of potential health hazards. They do not garauntee protection for all workers, nor do they carry any mandatory force.

South Africa has used TLVs in the past and still does for substances that do not have OELs. When evaluating air monitoring results it is vital to compare the OELs with TLVs and to use the one with the lower limit, as that will provide more protection for workers.

It is important to understand that there are limitations associated with use of exposure standards in evaluating working conditions:

• Although thousands of hazardous chemicals are used in industry, there are only exposure standards for about 700 chemicals. It is useful to note that just because a chemical does not have an OEL, that does not mean it is safe. This may only imply that his not been tested.
• There is a time delay between generation of information from toxicology studies and incorporation of that information into legal standards. Hence current OELs tend typically to be outdated.
• When developing standards governments are lobbied by various interest groups. This, more often than not, results in government agencies adopting compromised standards that are less effective in preventing harm to workers.
• Occupational Exposure Limits relate to inhalation as the route of entry into the body. This may underestimate the potential impact of the hazardous substances especially if other routes of entry into the body are significant.

There are other issues related to occupational hygiene standards that will not be discussed further. These include availability of laboratory techniques and use of OELs in environmental exposure.

BIOLOGICAL MONITORING:

Environmental monitoring has limitations as far as evaluation of workers' health risks is concerned. ⁵ These include failure to account for variation in health status of workers and how effectively the body deals with absorbed contaminants. Also, workers adopt different work and hygiene habits, and these influence the overall body burden of absorbed contaminants. Air monitoring does not take into account exposure to mixtures of contaminants (unless all components are monitored simultaneously). Skin and gastro-intestinal absorption may form a significant part of the overall exposure.

In order to take this into account, there is a need to have an integrated measure of how much contaminant has effectively entered the body for individual workers. Biological exposure monitoring involves the measure of changes in the composition of body fluids, tissues or exposed air to determine absorption of a potentially hazardous substance.

Biological exposure limits have been developed against which the biological exposure measurements are compared, although progress has been limited. The American Conference of Governmental Industrial Hygienists has published a list of Biological Exposure Indices (BEIs). According to ACGIH the BEIs are intended to complement rather than to replace air monitoring. Table 3 of the Regulations for Hazardous Chemical Substances contains a list of reference values which should be used when evaluating health risks for certain substances.

When using BEIs to evaluate health risks it is important to bear in mind that they have not been designed to measure extent of adverse health effect (this is the area of biological effect monitoring or clinical effect surveillance) nor to diagnose occupational diseases.