Automotive Emissions

AUTOMOTIVE EMISSIONS

Pollutant emissions from transportation sources, particularly automobiles and trucks, make up a substantial portion of national and global air pollution. They are major sources of carbon monoxide, nitrogen oxides, polycyclic organic matter, and volatile organic compounds (VOCs), such as benzene, a known cause of human leukemia and a subject of long-term regulatory concern. Nitrogen oxides and VOCs in automotive emissions are also major precursors for the formation of tropospheric ozone, which is of particular concern because there are substantial portions of the United States that experience adverse health consequences due to summertime oxidant smog, of which ozone is the key component. Diesel fuels, primarily used by trucks, also contribute to the overall particulate load as well as to mutagenic polycyclic aromatic hydrocarbons (PAHs). Resolving the current controversy about the role of diesel exhaust in causing cancer, as well as noncancer endpoints such as asthma, is important as it will affect decisions about the future development of automotive engines. Automotive emissions are also major contributors to the production of carbon dioxide, a significant contributor to global climate change.

Many automotive emissions of concern have come from fuel additives. Recognition of the role of lead in gasoline in producing subtle effects on childhood neurological and neurobehavioral development has led to a phasing out of leaded gasoline in the United States, an approach now followed in much of the world. The blending with gasoline of methyl tertiary-butyl ether (MTBE), an oxygenated ether, is also currently being phased out in the United States due to concerns about health effects and groundwater contamination. Unfortunately, other potentially neurotoxic heavy metals, such as manganese, are still being considered as fuel additives, and there are other oxygenated ethers in use for which there is even less toxicological information than there is for MTBE.

Changes in the design and engineering of automobiles have led to significant reductions in emissions. Evaporative loss of gasoline hydrocarbons has declined; increased engine efficiency has reduced carbon monoxide emissions; and the catalytic converter has significantly limited tailpipe emissions of all pollutants, except for nitrogen oxides. However, the decreases in emissions per mile driven have barely kept up, if at all, with the marked increase in miles driven. Continuing worldwide increases in the number of automobiles powered by gasoline and diesel fuel are inevitable for at least a few decades. Developing countries routinely go through a transition to reliance on greater numbers of automobiles and trucks, usually preceded by a surge in motorcycles or mopeds powered by two-cycle engines, which are themselves heavily polluting. There is a longer-term possibility of lesser emissions from automobiles powered by electricity or by other less-polluting sources such as hydrogen. Hybrid (gasoline and electric) automotive power sources may make inroads relatively quickly.

Land planning is also a major determinant of automotive pollution emissions. Urbanization in developing countries and urban and suburban sprawl in developed countries both have adverse implications on automotive emissions that can be avoided though proper planning.

BERNARD D. GOLDSTEIN

(SEE ALSO: Airborne Particles; Ambient Air Quality [Air Pollution]; Atmosphere; Benzene; Carbon Monoxide; Climate Change and Human Health; Fuel Additives; Land Use; Urban Health; Urban Sprawl; Urban Transport)

BIBLIOGRAPHY

National Research Council (1996). Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels. Washington, DC: National Academy Press.

Sawyer, R. F. (1993). "Trends in Auto Emissions and Gasoline Composition." Environmental Health Perspectives 101(supp. 6):5–9.

Watson, A. Y.; Bates, R. R.; and Kennedy, D., eds. (1988). Air Pollution, the Automobile, and Public Health. Washington, DC: National Academy Press.