Use of Categorical Regression in the Definition of the Duration/Concentration Curve in the U.S. EPA's Acute Refefence Exposure (ARE) Methodology
The U.S. EPA's current draft ARE methodology offers three different approaches for derivation of health effects values for various chemicals and agents under inhalation exposure scenarios of < 24 hrs. These approaches, the NOAEL, benchmark concentration (BMC), and categorical regression (CatReg), differ as to how duration/concentration curves are generated to accommodate derivation of values at different durations (e.g.,15 min vs 1 hr vs 24 hr). Overall, the preferred procedure for any of the approaches is to use a pharmacokinetic model. When a model is not available, the default procedure with the NOAEL and BMC approaches is to assume concentration (C) and time (t) are related as per Cn x t = K , where K is a constant and n is generally assigned a value of 1. The CatReg approach, based on categorizing data by severity of effect, is capable of directly generating a duration/concentration curve; further, such curves may be generated for different categories of severity. This presentation will briefly review these approaches and provide insights on duration/concentration curves generated from severity categories using the CatReg approach and, by using common data sets, on curves generated by the CatReg and Cn x t approaches.