Air Toxics from Vehicles and Their Fuels

Motor vehicles emit several pollutants that EPA classifies as known or
probable human carcinogens. Benzene, for instance, is a known human
carcinogen, while formaldehyde, acetaldehyde, 1,3-butadiene and diesel
particulate matter are probable human carcinogens. Studies are underway
to determine whether other toxic substances are present in mobile source
emissions. For example, EPA and industry are investigating whether
oxygen-containing fuel additives such as methyl tertiary butyl ether (MTBE)
cause any adverse health effects. EPA is also working with the vehicle and
fuel industries to test motor vehicle emissions for the presence of dioxin.
EPA estimates that mobile (car, truck, and bus) sources of air toxics account
for as much as half of all cancers attributed to outdoor sources of air toxics.
This estimate is not based on actual cancer cases, but on models that predict
the maximum number of cancers that could be expected from current levels of
exposure to mobile source emissions. The models consider available health
studies, air quality data, and other information about the types of vehicles
and fuels currently in use. Nonroad mobile sources (such as tractors and
snowmobiles) emit air toxics as well.