EPA has released its "Guidance for Applying Quantitative
Data to Develop Data-Derived Extrapolation Factors for Interspecies and Intraspecies
Extrapolation” (DDEF Guidance). This guidance document describes methods for calculating interspecies and intraspecies uncertainty factors (UFs). In the absence of chemical-specific data (such as toxicokinetic and/or toxicodynamic properties), default values for this variability are used which may be overly conservative. This guidance now brings EPA methodology more in line with both the 2005 World Health Organization’s International Programme on Chemical Safety approach as well as a 2011 policy establishing "body weight scaling as the default method for the interspecies toxicokinetic extrapolation for orally encountered contaminants."