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External Review Draft: Harmonization in Interspecies Extrapolation: Use of BW 3/4 as Default Method in Derivation of the Oral RfD

This science policy paper is in response to the Agency's Risk Assessment Forum's effort to development and promote harmonized approaches for all toxicity endpoints used in human health risk assessment. Towards the goal of harmonization, the specific purpose of this science policy paper is to recommend procedures, inclusive of default procedures, that will be used to estimate oral exposures in terms of human equivalents for all toxicologic endpoints. In doing so this work is intended to follow and be concordant with the mode-of-action as the guiding paradigm for toxicologic evaluations, both cancer and noncancer (USEPA 2005; USEPA 1994; USEPA 2002). That the dose at the target site, the internal dose, "is the ultimate determinant of risk" (NRC, 1994), is a fundamental generalization of this paradigm. 
 
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