The Clean Air Act identifies 189 hazardous air pollutants (HAPs), or "air t
oxics," associated with a nide range of adverse human health effects. The U
.S. Environmental Protection Agency has conducted a modeling study with the
Assessment System for Population Exposure Nationwide (ASPEN) to gain a gre
ater understanding of the spatial distribution of concentrations of these H
APs resulting from contributions of multiple emission sources. The study es
timates year 1990 long-term outdoor concentrations of 148 air toxics for ea
ch census tract in the continental United States, utilizing a Gaussian air
dispersion modeling approach. Ratios of median national modeled concentrati
ons to estimated emissions indicate that emission totals without considerat
ion of emission source type can be a misleading indicator of air quality. T
he results also indicate priorities for improvements in modeling methodolog
y and emissions identification. Model performance evaluation suggests a ten
dency for underprediction of observed concentrations, which is likely due,
at least in part, to a number of limitations of the Gaussian modeling formu
lation. Emissions estimates for HAPs have a high degree of uncertainty and
contribute to discrepancies between modeled and monitored concentration est
imates. The model's ranking of concentrations among monitoring sites is rea
sonably good for most of the gaseous HAPs evaluated, with ranking accuracy
ranging from 66 to 100%.