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Data Summaries and Sources


Data Summaries:
The 1999 National Emissions Inventory (NEI) is the underlying basis for the 1999 emissions used in the national-scale assessment. For purposes of the assessment, EPA divided the emissions estimates into four overarching source types: major, area and other, onroad mobile, and nonroad mobile. In preparing the inventory for modeling, EPA used default, or simplifying, assumptions where data were missing or of poor quality. For example, sometimes data on stack height or facility location were not available or were flawed. When this happened, they were replaced by default assumptions. For example, a stack height for a facility might be set equal to stack heights at comparable facilities; the location of the facility might be placed at the center of a census tract; etc.

For additional information about 1999 emissions of air toxics, including sources of emissions, visit AIRData.

For summary information on air pollution trends, including air toxics trends, visit EPA's Air Trends Web site.

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Data Sources:
EPA compiled the 1999 National Emissions Inventory (NEI) using five primary sources of data:

In compiling stationary source emissions information for the NEI, preference is given to State- and locally-generated information where available. Where such data are not available, existing data from EPA's regulatory development databases are utilized. If neither of these data sources contains information for a known stationary source, EPA uses data from the TRI. EPA also gives preference in inventory development to emissions data resulting from direct measurements over those generated from emissions factors and activity data.

For more information on emission inventories, see the National Emission Inventory Data web page.

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