US EPA

The possibility of nonlinear effects of body weights should be considered in any analysis of biological parameters across species with significantly different body sizes. The term “allometry” is used where there is a possibly non-linear relationship of a toxicologically relevant parameter to body weight (BW), particularly when the relationship can be described with a power function BW^b with exponent b. (We also use the term “scaling”.) This report discusses scaling defaults for terrestrial wildlife oral toxicity measurements in the form of exposure values (dose or concentration) associated with specified toxicological outcomes. These may be in “dietary” form (e.g., ppm toxicant in feed) and dose form (e.g., mg/kg or mg/kg-d).

Extrapolation of dose-based toxicity measurements on a “simple body weight basis” (the case b = 1) is held to apply primarily to lethality measurements based on single doses, or other situations involving appreciable lethality, while in most other situations an allometric adjustment based on b = ¾ is the recommended default. The default for a “dietary” or (“food based”) toxicity, reported and applied as toxicant concentration in feed, is no allometric scaling. Exceptions for particular situations can be based on direct empirical evidence, mechanistic information, or modeling. The recommendations are consistent with current health assessment policy (U.S.EPA, 2011).

Due to uncertainty surrounding the use of allometry in ecological risk assessments, ERASC was requested by risk assessors in the EPA Office of Land and Emergency Management (OLEM) program to clarify the appropriate use of allometric scaling of toxicity measurements in ecological risk assessments.

This is the final report.

U.S. EPA. Allometric Scaling of Terrestrial Wildlife Oral Toxicity Measurements and Comparison of Ecological to Human Health Assessment Contexts. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-21/305, 2021.