Mobile source air toxics are compounds emitted from highway vehicles and non-road equipment which are known or suspected to cause cancer or other serious health and environmental effects. Mobile sources are responsible for direct emissions of air toxics and contribute to precursor emissions which react to form secondary pollutants. Examples of mobile source air toxics include benzene, 1,3-butadiene, formaldehyde, acrolein, polycyclic organic matter, naphthalene, and diesel particulate matter.
In February 2007, EPA finalized the Control of Hazardous Air Pollutants from Mobile Sources rule to reduce hazardous air pollutants from mobile sources. The rule limits the benzene content of gasoline and reduces toxic emissions from passenger vehicles and gas cans. EPA estimates that by 2030 this rule will reduce total emissions of mobile source air toxics by 330,000 tons and VOC emissions (precursors to ozone and PM2.5) by over 1 million tons. EPA adopted mobile source emissions control programs that, in addition to controlling pollutants such as hydrocarbons, particulate matter, and nitrogen oxides, also result in large air toxic reductions.
The chart below shows the source contributions for air toxics. On-road vehicles account for about one-quarter of all air toxic emissions.
National Contribution of Each Emissions Source Type (2011)
Source: U.S. Environmental Protection Agency, http://www.epa.gov/orcdizux/toxics.htm
The EPA 1999 National Air Toxics Assessment (NATA) identified seven compounds with significant contributions from mobile sources that are among the national and regional-scale cancer risk drivers:
Some of these chemicals (such as benzene) are present in gasoline and diesel fuel and are emitted through evaporation or when fuel passes through an engine without being burned. Others (such as formaldehyde and diesel particulate matter) are not present in the fuel itself; rather, they are byproducts of incomplete combustion. These compounds have a variety of potential human health effects. Benzene, for instance, is a known carcinogen.
The chart below shows the estimated proportion of emissions from the seven notable MSATs from on-road vehicle activity in 2015.
Distribution of MSATs Emissions from On-Road Vehicles (2015)
Source: Federal Highway Administration, http://www.fhwa.dot.gov/environment/air_quality/air_toxics/policy_and_guidance/aqintguidmem.cfm
The following chart shows the estimated on-road reductions in seven MSATs through 2050 despite an expected 102 percent increase in national VMT. The decreases are derived from improved vehicle technology and emissions modeling tools.
Estimated Percent Change in MSAT Emissions (2010–2050)
Trends for specific locations may be different, depending on locally derived information representing vehicle-miles travelled, vehicle speeds, vehicle mix, fuels, emissions control programs, meteorology, and other factors.
Sources: Federal Highway Administration memorandum, Interim Guidance Update on Mobile Source Air Toxic Analysis in NEPA, December 2012, http://www.fhwa.dot.gov/environment/air_quality/air_toxics/policy_and_guidance/aqintguidmem.cfm#fig1