US EPA

Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs) are commonly found as contaminants in complex mixtures in the environment. Several of these compounds bioaccumulate in the tissues of fish, birds, and mammals and have been shown to cause mortality and adverse effects on reproduction, development, and the immune system.

For more than a decade, 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 the amount of data and experience with this methodology have increased, experts have come to the consensus that the toxicity equivalence methodology can strengthen assessments of ecological risks.

Organized in accordance with EPA's Guidelines for Ecological Risk Assessment, this framework is intended to assist EPA scientists in using the toxicity equivalence methodology in ecological risk assessments that involve dioxins and dioxin-like chemicals, as well as to inform EPA decision makers, other agencies, and the public about this methodology.

While this framework is organized around EPA's Guidelines for Ecological Risk Assessment, it is not intended to be a comprehensive guide to risk assessment involving dioxin-like chemicals. Rather, this framework summarizes the toxicity equivalence methodology; provides definitions and a description of how the methodology has evolved; provides ecological risk assessors with an understanding of issues to consider when applying the methodology in ecological risk assessments; and summarizes the benefits, implications, and uncertainties of the methodology within the context of ecological risk assessments.

The Ecological Toxicity Equivalence Factor (Eco-TEF) framework is intended for guidance only. It does not establish any substantive "rules" under the Administrative Procedure Act or any other law and will have no binding effect on EPA or any regulated entity. Rather, it represents a nonbinding statement of policy. EPA believes that this framework provides a sound, up-to-date presentation of a method for use in conducting risk assessments involving dioxins and dioxin-like chemicals, and serves to enhance the application of the best available science.