Assessment of estimated 1990 air toxics concentrations in urban areas in the United States

The 188 air contaminants designated as hazardous air pollutants, or air toxics, under the Clean Air Act Amendments of 1990 are associated with a variety of adverse human health impacts. The US Environmental Protection Agency recently developed estimates of 1990 outdoor concentrations of 148 air toxics for every census tract in the continental United States. This paper compares the results for urban and rural areas, and evaluates the relative contributions of large stationary sources (point sources), small stationary sources (area sources), and mobile sources. The estimated air toxics concentrations in urban areas were typically twice as high as in rural areas. There were more air toxics with modeled ambient concentrations in excess of health benchmarks in urban census tracts than in rural census tracts. Ambient concentrations attributable to area sources alone exceeded health benchmarks in a majority of urban census tracts for several pollutants; similar results were found for mobile sources. For point sources, exceedances of benchmarks generally occurred in fewer census tracts. These results show that reductions in emissions of air toxics from all three types of sources will be necessary to reduce anthropogenic air toxics concentrations to levels below the health benchmark concentrations.