Air Pollutants
Box 2

Mobile Source vs. Transportation Emissions

The U.S. Environmental Protection Agency’s (EPA) National Emission Inventory (NEI) is updated annually and covers both mobile and stationary sources of pollution. The mobile portion contains “onroad” (highway) and “nonroad” (all other modes) emissions. However, the nonroad category also contains nontransportation sources such as farming and construction equipment, lawn and garden equipment, and logging, industrial, and light commercial equipment. While the Bureau of Transportation Statistics uses the mobile portion of the NEI to characterize transportation air emissions, it does so by first removing the nontransportation components of the data. Thus, data presented here cannot be directly compared with the EPA NEI data. For instance, the mobile source component of all carbon monoxide emissions in 1999 was 77 percent, nitrogen oxides were 56 percent, and volatile organic compounds were 47 percent.