Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs) are commonly found as contaminants in complex mixtures in the environment, including in animal tissues. For more than a decade, the U.S. Environmental Protection Agency (EPA) and other organizations have estimated the combined risks that such mixtures pose to human health using a method known as the toxicity equivalence methodology. Application of this methodology in ecological risk assessments has proceeded more slowly, in part because of the variety of species from different taxonomic classes (e.g., fish, birds, and mammals) that need to be considered. As both data and experience with the methodology have accumulated experts have come to the consensus that the toxicity equivalence methodology can strengthen assessments of ecological risks. Consultations between EPA and the Department of Interior (DOI) on water quality criteria, based on 2,3,7,8- TCDD alone, for protecting endangered species in the Great Lakes led these agencies to more intensively explore the application of the toxicity equivalence methodology in ecological risk assessment. In 1998, EPA and DOI sponsored a workshop that recommended the development of further guidance on application of the toxicity equivalence methodology. This framework has been developed in direct response to that workshop recommendation.

Prepared under the auspices of EPA's Risk Assessment Forum. "EPA/100/R-08/004." "June 2008." Includes bibliographical references (pages 62-76).

Introduction -- Definitions -- Evolution of the toxicity equivalence methodology -- Toxicity equivalence methodology -- AHR-mediated mechanism and assignment of relative potency -- Selection of the appropriate relative potency factors -- Toxicity equivalence concentration -- Application of the toxicity equivalence methodology in ecological risk assessment -- Considerations in planning -- Considerations in problem formulation -- Considerations in analysis -- Considerations in risk characterization -- Conclusions.