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As part of its effort to address risks posed by chlorinated dibenzo-p-dioxins and dibenzofurans(CDDs and CDFs) in the environment, EPA has adopted an interim procedure, based on dioxin "toxicity equivalence" factors (TEFs), for estimating the hazard and dose-response of complex mixtures containing CDDs and CDFs in addition to 2,3,7,8-TCDD. The TEF procedure, and the scientific data upon which it is based, are the subject of this report. In summary, the TEF approach provides a useful interim method for consistently interpreting the significance of CDD and CDF residues in the environment, until more direct methods are available. Users should be aware of the uncertainties associated with this procedure. In addition to the uncertainties inherent in the 2,3,7,8-TCDD quantitative risk assessment, which the TEF approach implicitly adopts, the approach includes the added qualitative assumption that the other CDDs and CDFs will demonstrate the same chronic effects as 2,3,7,8-TCDD. While there are good scientific reasons to expect this to be the case, the data to support the assumption are limited.
The TEF procedure will be used generally throughout the Agency for situations in which the components of the mixture are known (or can be reasonably anticipated) and where the composition is not expected to vary greatly with time.