Background concentrations are the concentrations of HAPs measured at "clean" air locations remote from the impact of mobile or stationary emission sources. In the National-Scale Air Toxics Assessment (NATA), background concentrations were identified for 12 hazardous air pollutants (listed below) from published journal articles, and other publications. For these 12 HAPs, the estimated total concentration in any area is determined by summing together the estimated background concentrations and the modeled concentrations arising from 1996 emissions. Background concentrations are assumed to be the same across the entire U.S.; available data are insufficient to address potential geographic variations in background concentrations.
Benzene 0.48 µg/m3
Carbon Tetrachloride 0.88 µg/m3
Chloroform 0.083 µg/m3
Ethylene Dibromide 0.0077 µg/m3
Ethylene Dichloride 0.061 µg/m3
Formaldehyde 0.25 µg/m3
Hexachlorobenzene 0.00009 µg/m3
Methylene Chloride 0.15 µg/m3
Polychlorinated Biphenyls 0.00038 µg/m3
Tetrachloroethylene 0.14 µg/m3
Trichloroethylene 0.081 µg/m3
Xylene 0.17 µg/m3
NOTE: For diesel emissions, NATA estimates area-specific background concentrations. Mobile background concentrations were calculated by applying a medium-range pollutant transport model to all onroad and offroad mobile emissions sources located between 50 km and 300 km from each census tract centroid.