OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 4 Chemical Hazards, Biological Hazards and Harmful Substances

Section 17 Exposure to multiple substances

Workers are often exposed to a mixture of chemicals rather than a single substance. However, exposure standards or limits are usually based on information, testing or experience from exposure to a single chemical rather than a mixture of chemicals. The resulting biological effects of exposure to multiple chemicals is rarely known.

The combined effects of chemicals are commonly described as follows:

(a) Independent the toxicity of each substance is produced by independent mechanisms and/or the substances act on separate organs or systems within the body. Independent substances create their own toxic effects without influence or interference from one another.

(b) Additive substances with similar toxicity produce a response that is equal to the sum of the effects produced by each of the individual substances acting alone.

(c) Antagonistic the toxicity of one chemical is reduced by exposure to another chemical.

(d) Potentiating – a substance does not have a toxic effect on a certain organ but when combined with exposure to another substance, the first substance becomes much more toxic.

(e) Synergistic – two substances act together to produce toxic effects that are greater than the effects produced by either substance alone.

In evaluating the impact of exposure to more than one chemical at a time, materials acting independently can be evaluated independently. Where the potential for synergistic or potentiating effects are suspected, this enhancement of toxic effect must be reflected in the allowable exposure. There is no model for adjustment of exposure limits to account for synergistic or potentiating effects. The easiest solution is to avoid the effect by finding a substitute for one or more of the chemicals involved. In occupational settings, antagonistic effects are not used as a basis for decreasing exposure limits.

Where chemicals are known to have additive effects, the equation provided in this section allows the employer to determine whether the OEL is being exceeded. To prevent overexposure, the sum of the standardized exposures must not exceed the value “one”. A mixture of xylene and toluene is an example of two substances that produce additive effects; a mixture of xylene and asbestos is an example of two substances that produce independent effects. The equation can be used in the first example, not in the second.