Relatively little is known about the spectrum of health effects, and the scope and level of ambient air concentrations of those pollutants regulated under the Clean Air Act as "hazardous air pollutants". The U.S. Environmental Protection Agency's (USEPA) Cumulative Exposure Project uses currently available emissions inventories, from a variety of source types, and an atmospheric dispersion model to provide estimates of ambient concentrations for 148 hazardous air pollutants (HAPs) in over 60,000 census tracts for the year 1990. This paper uses currently available hazard information for those pollutants and provides a database of potential regulatory threshold concentrations of concern, or "benchmark concentrations," and a methodology for prioritizing and characterizing the quality of the data. In order to demonstrate application of the database and prioritization scheme to outputs from the Cumulative Exposure Project, comparisons were made with the maximum modeled concentration of each individual hazardous air pollutant across the census tracts. Of the 197 benchmark concentrations for cancer and non-cancer (long- and short-term exposures) effects compiled for the study, approximately one half were exceeded with a predominance of exceedance of cancer benchmarks. While the number of benchmark concentrations available to fully characterize potential health effects of these pollutants was limited (approximately 80 percent of HAPs identified as cancer concerns had benchmark concentrations for cancer and 50 percent of all HAPs had non-cancer benchmark concentrations) and there was greater uncertainty in derivation of maximum modeled air concentrations than other levels, the comparison between the two was a useful approach for providing an indication of public health concern from hazardous air pollutants.