While some potential environmental hazards involve significant exposure to only a single compound, most instances of environmental contamination involve concurrent or sequential exposures to a mixture of compounds that may induce similar or dissimilar effects over exposure periods ranging from short-term to lifetime. Multichemical exposures are ubiquitous, including air and soil pollution from municipal incinerators, leakage from hazardous waste facilities and uncontrolled waste sites, and drinking water containing chemical substances formed during disinfection. To address concerns over health risks from multichemical exposures, EPA issued Guidelines for Health Risk from Exposure to Chemical Mixtures
in 1986. Those Guidelines described broad concepts related to mixtures exposure and toxicity and included few specific procedures. Since then, EPA has published additional mixtures guidance documents such as 1989 guidance for the Superfund program on hazardous waste and the 1990 Technical Support Document on Health Risk Assessment of Chemical Mixtures. Because the science of environmental risk assessment continues to evolve, EPA's Risk Assessment Forum established a Technical Work Panel to ensure that the advances in the area of chemical mixtures health risk assessment are reflected in the Agency's guidance materials. This document has been developed by the Technical Work Panel to supplement the earlier guidances and is organized according to the type of data available to the risk assessor, ranging from data rich to data poor situations. Procedures are described for assessment using data on the mixture of concern, data on a toxicologically related mixture, as well as data on the mixture component chemicals. No single approach is recommended in this supplementary guidance. Instead, guidance is given for the use of several approaches depending on the nature and quality of the data.