National data on air emissions are estimated by the U.S. Environmental Protection Agency (EPA). EPA's National Emissions Inventory (NEI) is updated annually and covers mobile, stationary, and area sources of pollution regulated under the Clean Air Act. These pollutants include the so-called "criteria" and hazardous air pollutants.1 Most criteria emissions have been estimated since 1970, hazardous emissions only since 1996.
EPA's mobile source category contains "onroad" (highway) and "nonroad" (all other modes) emissions. However, its nonroad category includes nontransportation sources such as farming and construction equipment, lawn and garden equipment, and logging, industrial, and light commercial equipment. To more accurately assess transportation air emissions, the Bureau of Transportation Statistics removes the nontransportation components from EPA's criteria mobile source emissions. It is this subset that is presented here as transportation emissions.
1 For details on transportation's contribution to hazardous air pollutants, see U.S. Department of Transportation, Bureau of Transportation Statistics, Transportation Statistics Annual Report 2000, available at http://www.bts.gov/, as of August 2005.