Hazardous air pollutants (HAPs) are chemicals which can cause adverse effects to human health or the environment. Congress has identifed over 188 of these pollutants, including substances that cause cancer, neurological, respiratory, and reproductive effects. Recent studies have detected HAPs at concentrations that warrant public health concern in many cities. Congressional reports on exposure to HAPs in Los Angeles and the San Francisco Bay Area determined that the estimated cancer risks posed by current monitored concentrations of HAPs in those regions exceed the Clean Air Act's health goals by more than a factor of 100. In most areas, high concentrations of HAPs are primarily due to pollution sources like trucks or small businesses that have not had the same regulatory scrutiny as large industrial facilities. Moreover, a very small number of chemicals and chemical categories appear to account for the majority of health risks associated with hazardous air pollutants - in particular, diesel emissions from vehicles and equipment.
DATA SOURCES AND LIMITS
Scorecard combines exposure estimates from U.S. EPA's National-Scale Air Toxics Assessment with toxicity data to estimate the health risks posed by chemical pollutants in ambient air. The National-Scale Air Toxics Assessment (NATA) estimates annual average outdoor concentrations of 41 hazardous air pollutants, using 1996 emissions data. It is necessary to rely on exposure estimates rather than actual monitoring because there are fewer than 50 monitoring stations measuring outdoor levels of HAPs in the entire country. NATA currently provides the only data available for assessing the extent of exposures to hazardous air pollutants across the entire U.S. Scorecard uses conventional risk assessment methods to produce a screening-level health risk assessment, which offers valuable information about the magnitude and sources of hazardous air pollution problems.
There are important uncertainties that affect predictions about health risks based on estimated exposure data. Please review our Caveats. EPA cautions against "using the results of this modeling exercise alone to draw real-world conclusions about current local conditions" because of the limitations involved in modeling exposures using 1996 emissions data. There have been changes in emissions since 1996 in pollution sources and quantities that may affect the reliability of EPA's exposure estimates.
EPA has compared NATA estimates with actual 1996 monitoring data and concluded that its "modeled estimates for most of the pollutants examined are typically lower than the measured ambient annual average concentrations." NATA exposure estimates are more consistent with current air monitoring data. Scorecard compares NATA estimates with current monitoring data from several states, and provides online access to these data whenever they are available. Since EPA's exposure estimates tend to underpredict ambient concentrations and do not address all toxic air pollutants, the potential health risks from HAPs may be underestimated.
Census tract-level concentration data and source apportionment data were obtained from a CD-ROM distributed by EPA's Office of Air Quality Planning and Standards in February 2001. County-level emissions data were obtained from EPA's NATA website. Scorecard utilizes the 2000 Census for data on the population in areas covered by NATA.
Several states and territories are not included in the Scorecard's hazardous air pollution profiles due to data limitations. NATA exposure data are not available for Alaska and Hawaii. 2000 census data are not available for Puerto Rico and the Virgin Islands, although EPA does provide exposure data for these areas.