ESTIMATING HUMAN EXPOSURE TO SELECTED MOTOR-VEHICLE POLLUTANTS USING THE NEM SERIES OF MODELS - LESSONS TO BE LEARNED

This paper reviews the use of exposure modeling by the Ambient Standar ds Branch (ASB) of EPA's Office of Air Quality Planning and Standards. The Branch uses exposure assessments to evaluate health risks associa ted with attainment of alternative National Ambient Air Quality Standa rds (NAAQS). This paper examines the history of the NAAQS Exposure Mod el (NEM) and probabilistic NEM (pNEM) models and the role that they ha ve played in NAAQS reviews of lead, carbon monoxide, nitrogen dioxide, and oxygen. Trends in how the following substantive issues were addre ssed in the NEM series of models are reviewed: (1) exposure and dose m etrics; (2) microenvironmental (mu e) concentration estimation; and (3 ) human activity and breathing rate simulation. In response to an outs ide peer review of its recent exposure assessments, ASB is deemphasizi ng modeling the entire population in favor of limited modeling of narr owly defined ''sensitive groups.'' In addition, ASB increasingly is fo cusing its exposure assessments on those human activities that lead to high intake dose, or high intake dose rate. Examples are provided tha t highlight these changes in emphasis.