Current and future risk assessment guidelines, policy, and methods development for chemical mixtures

Humans are typically exposed to low doses of combinations of chemicals rather than to one or two chemicals at a time, yet most of the available toxicity data provide information on single chemicals or binary pairs, rather than on whole mixtures. The use of existing interactions study data for the quantitative risk assessment of chemical mixtures is problematic. These studies generally lack the necessary statistical characterizations to be useful in quantitative risk assessment procedures. The U.S. EPA developed guidelines for risk assessment for chemical mixtures in 1986 and is currently in the process of making revisions. Significant advances have been made in both the theoretical development and application of procedures such as dose addition, response addition, toxicity equivalence factors, comparative potency and interactions data characterizations. Details on the current revisions to the guidelines are given, along with information on the research efforts that have influenced these revisions or that represent future directions in chemical mixtures risk assessment.