Comparison of BMD with NOAEL and LOAEL Values Derived from Subchronic Toxicity Studies

Health-based exposure standards for non-cancer endpoints are currently derived based on a premise that an acceptable level can be determined at which there will be no significant risk of an adverse human health effect. This assumes the existence of a threshold exposure level for the adverse effect in question. The approach most widely used for non-cancer endpoints in the United States is to determine the no-observed-adverse-effect-level (NOAEL) for the critical effect and then to apply uncertainty factors (UFs) to account for scientific uncertainties in the total data base (Barnes and Dourson, 1988). The resulting value is referred to as a reference dose (RfD) for oral exposure, or reference concentration (RfC) for inhalation exposure. This approach is similar to other methods used worldwide, e.g., derivation of Acceptable Daily Intake or the ADI.