Why is EPA using computer modeling techniques instead of actual measurements to estimate concentrations and exposure?
The assessment approach is fundamentally based on using computer models to estimate ambient air toxics concentrations and population exposures nationwide. While such computer models necessarily require simplifying assumptions and introduce significant uncertainties, they are needed to conduct such a large scale assessment since direct measurements of ambient air toxics concentrations are limited, and direct personal exposure measurements are even more limited. Such measurements are available for only a subset of air toxics in relatively few locations and for small study populations. Although EPA is working to expand the number and locations of ambient air toxics monitors and the study of personal exposures, direct measurement of air toxics concentrations is not practical for all air toxics of interest across all areas of the country. Over time, such measurement data can and will be used, however, to evaluate the models so as to better understand some of the uncertainties in such assess
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