The exposure standards listed in this document are applicable to airborne concentrations of single pure substances. In practice, however, a working environment may contain a number of airborne pollutants and exposure to these additional substances, either simultaneously or sequentially, could give rise to an increased hazard to health.

Although the following proposals have been made to deal with mixtures, in several specific cases, the application of exposure standards to environments containing mixtures of pollutants requires considerable caution. The interaction of any particular mixture of substances should be assessed by either toxicologists, occupational hygienists or physicians after specific toxicological consideration of ail substances involved:

Where there is clear toxicological evidence to indicate that two or more pollutants have totally distinct mechanisms of effect on the body, each substance may be separately evaluated against its appropriate exposure standard, For example, since crystalline silica affects the lungs, and inhaled ethanol vapour acts upon the liver and central nervous system, each of these substances may be assessed individually against its appropriate exposure standard, if neither standard is exceeded, the atmosphere within the working environment is deemed to be satisfactory.

When the body is exposed to two or more contaminants, an additive effect is obtained when pollutants have the same target organ or the same mechanism of action. In this situation, the total effect upon the body equals the sum of effects from the individual substances.

When assessing the hazard from a mixture of airborne contaminants, it is important to identify and quantify all components in the airborne mixture as a number of factors, such as particle size distribution or solvent vapour pressure, can give rise to substantial variations between the concentration of each component in the parent mixture and that which occurs in air.

Although an example of an additive effect is the general effect of organic solvents on the central nervous system (narcotic or anaesthetic effect), the exposure standard for a number of solvents, such as benzene and carbon tetrachloride, have been assigned on the basis of effects other than those on the central nervous system.

Therefore, it is essential to refer to the documentation for the specific substances to ascertain the basis of the standard and any potential interactions.

Sometimes the combined effect of multiple exposures is considerably greater than the sum of the effects from the individual components. This phenomenon can be one of synergism or potentiation. Synergism occurs when both chemicals have an effect individually and a more than additive effect when together. Potentiation is when one chemical has an effect, but the second chemical does not, but enhances the effect of the former chemical on combined exposure.

An example of a synergistic effect is the combined effect of solvents such as n-hexane and methyl ethyl ketone (MEK) on the nervous system. In combination, the damage caused by simultaneous high concentrations of both these solvents is far greater than the sum of either of these substances acting alone.

Interaction can also arise from exposures via routes other than inhalation. For example, imbibed alcohol increases the narcotic effects of inhaled trichloroethylene. Interaction effects may also occur in connection with exposure to entirely different environmental factors such as simultaneous exposure to chemical agents and physical factors, such as light, heat and noise. Smoking of tobacco is known to have a synergistic effect in combination with for example, inhaled particulates.

At present the understanding of interaction effects is incomplete. The knowledge that such effects can occur is reason to maintain the concentrations of individual substances as low as is practicable under complex exposure conditions.

TABLE C1: Substances which are considered to have additive or synergistic effects