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Summary of the Emissions and Ambient Concentration Modeling Results

  • The distribution of emissions and concentrations does not necessarily correlate directly with risk; we will be addressing the risk distribution in the next phase of the assessment.


  • Concentration estimates are a complex function of a number of factors, including emissions density (number of sources in a particular area), meteorology, and source characteristics, rather than just related to total emissions.


  • Both emissions and estimated concentrations of the 32 air toxics available to date are generally higher in urban than in rural areas.


  • Some pollutants are more evenly distributed around the country (e.g., benzene, which is present in gasoline) while others are linked to areas of industrial activity (e.g.,vinyl chloride).


  • There is considerable variability between the national, state and the county level in terms of contributions by source type.


  • Because different types of sources are contributing to emissions in different areas of the country, the highest ambient average concentration of the individual pollutants occurs in different States (i.e., no one State has the highest concentrations of all the pollutants).


  • The background concentration consists of contributions to outdoor concentrations resulting from natural sources, persistence in the environment, and long-range transport. EPA has background estimates for 13 of the 33 air toxics. For 7 of these 13 pollutants (PCBs, ethylene dibromide, carbon tetrachloride, hexachlorobenzene, ethylene dichloride, chloroform, and mercury), the background dominates the total estimated average concentration.


  • Of the four main source types (area and other, major, onroad, non road), no one type is a main contributor to the estimated concentrations of the 32 pollutants available to date. The results show that, on a national level, about half of the pollutants have "area and other sources" as the dominant contributing source type. For more details about which source type is the dominant contributor to concentrations for each pollutant, see table.

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