Researchers linked automobile use to air pollution in the early 1950s when A. Haagen-Smit of the California Institute of Technology and fellow researchers began to unravel the complex atmospheric chemistry that leads to the formation of photochemical smog (ozone). Ozone is a strong lung and throat irritant that decreases lung function, increases respiratory problems, and complicates heart disease. Moderate ozone concentrations also damage materials and crops, increasing the cost of living. Ozone forms in the atmosphere when oxides of nitrogen (NO_x) and hydrocarbons (HC) mix and react in the presence of sunlight. Because onroad automobiles, trucks, passenger vans, and sport utility vehicles are typically responsible for about 30 percent of a region's HC and NO_x emissions, transportation is a significant contributor to smog problems in urban areas.

Onroad transportation sources are also responsible for more than 60 percent of regional carbon monoxide emissions. Carbon...