While the U.S. Environmental Protection Agency (EPA) has made annual estimates of emissions of six principal (so-called, criteria) air pollutants covering several decades, it has only recently concentrated on the larger set of hazardous (toxic) air pollutants. Two datasets on toxics are available: a 1990 to 1993 baseline and 1996. Data for 1999 may be available in 2002.
In 2000, EPA designated 21 of the 188 hazardous air pollutants (HAPs) as mobile source air toxics (MSATs). Among these, mobile sources contribute 86 percent of the nation's air emissions of methyl-tertiary-butyl-ether (MTBE), 84 percent of ethylbenzene, 79 percent of xylene, 76 percent of acetaldehyde, and 60 percent of 1,3-butadiene.1 EPA published a final rule on controlling MSAT emissions in March 2001.2
1 The Bureau of Transportation Statistics published the 1996 data on 21 MSATs in its Transportation Statistics Annual Report 2000 (Washington, DC: 2002), also available at http://www.bts.gov.
2 66 Federal Register 17229, Control of Emissions of Hazardous Air Pollutants from Mobile Sources, Mar. 29, 2001.