Risk assessment of chemical mixtures from a public health perspective

Health risk assessment is the practice of evaluating the degree of danger associated with chemical exposure, whether the exposure is intentional (pharmacologic agents, pesticides) or unintentional (industrial/automobile by-products). Chemical exposure can either be to a single chemical or to complex mixtures such as industrial effluents, municipal wastes, jet fuels, gasoline, or mixtures of drinking water contaminants. The mixtures can be simple or complex; partially or completely characterized; and stable or varying in composition. Three different approaches are often used in health risk assessment of chemical mixtures (51 FR 33992-34054). These 3 approaches consist of (a) use of data on the specific mixture of concern; (b) use of data on a similar mixture; and (c) use of data on each component of the mixture. The individual component-based approach is by far the most often used because it allows the individual risks from each component to be combined, usually by dose or response additivity, to calculate an overall risk for the mixture. In addition, several innovative methods, such as the toxicity equivalency factor, relative potency, and even the use of indicator chemicals, are also employed. More recently, a binary weight-of-evidence approach has been proposed to evaluate potential interactions between the various components and to integrate them into the overall toxicity assessment of the mixture. Because no single approach is suitable for assessing the health risk associated with all the exposure scenarios associated with the various types of mixtures, the use of professional judgment is still imperative in conducting health risk assessments.