Scorecard's data on emissions of criteria air pollutants are derived from EPA's National Emissions Trends (NET) database. The NET database contains emissions levels from mobile, area, and point sources for counties across the U.S. It provides detailed emissions data for over 50,000 point sources (industrial and other facilities).
Emissions data are currently available for 1999. EPA conducts a comprehensive emissions inventory every three years. EPA also modifies the NET database at least annually to correct and augment its inventory data. Currently the NET database has point source emissions data for year 1999, and county-level aggregate emissions from all source types for years 1985 - 1998.
EPA's methodology for constructing the NET database is described in its National Emissions Inventory (NEI) Air Pollutant Emissions Trends. States collect emissions data and report it to the EPA for compilation in the NET database, so data quality, completeness, and reporting years can vary from state to state. States sometimes use different methods to develop data for specific pollutants and sources (e.g., direct reporting vs. estimates based on emissions factors). Since not all states provide specific information for all sources, EPA must also sometimes estimate emissions to complete the data record for a state.
The NET database breaks all air pollution generating activities into Source Classification Codes (SCCs). These classifications are used to develop estimates and projections of air pollutant releases, and to identify the major pollution generating activities within specific source categories. More from EPA on SCCs.
The NET is an emissions database developed by EPA for its internal use. The NET is based partially on emission data obtained from state and local agencies, but it is not an official compilation of state emissions data. There are known inconsistencies between EPA's NET and official state emission inventories. Scorecard's emissions data were derived from a 2001 version of the NET database, which is incrementally modified by EPA to generate the online NET database. This online database captures corrections to facility or area criteria emissions data made by companies or state agencies.