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 Standards Branch (ASB) of EPA's Office of Air Quality Planning and Standards. The Branch uses exposure assessments to evaluate health risks associated with attainment of alternative National Ambient Air Quality Standards (NAAQS). This paper examines the history of the NAAQS Exposure Model (NEM) and probabilistic NEM (pNEM) models and the role that they have played in NAAQS reviews of lead, carbon monoxide, nitrogen dioxide, and oxygen. Trends in how the following substantive issues were addressed in the NEM series of models are reviewed: (1) exposure and dose metrics; (2) microenvironmental (mu e) concentration estimation; and (3) human activity and breathing rate simulation. In response to an outside peer review of its recent exposure assessments, ASB is deemphasizing modeling the entire population in favor of limited modeling of narrowly defined "sensitive groups." In addition, ASB increasingly is focusing its exposure assessments on those human activities that lead to high intake dose, or high intake dose rate. Examples are provided that highlight these changes in emphasis.