US EPA

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.

Gift, J. S., C. Liu, AND G. L. Foureman. Use Of Categorical Regression In The Definition Of The Duration/Concentration Curve In The U.S. EPA's Acute Refefence Exposure (ARE) Methodology. Presented at SRA, New Orleans, LA, Dec. 8-11, 2002.