The environmental consequences of oil use in conventional automobiles are staggering. Emissions from transportation now account for roughly 50% of all air pollution regulated under the Clean Air Act. Because automotive use is concentrated in cities, the total pollution due to transportation in most urban areas, including toxic air pollutants such as benzene and formaldehyde, exceeds 60%.²¹

Because automotive exhausts generally occur at ground level in populated areas, the human health effects of tailpipe emissions are enormous. Public awareness of the impact of transportation pollution is also high. The emissions of hydrocarbons and nitrogen oxides together are particularly disturbing because their interaction in the atmosphere causes ozone, or smog, which is the most critical air quality issue facing many cities. Particulate emissions, largely from the exhaust of diesel fuel engines, are also especially worrisome because they have been linked to severe respiratory problems, including premature deaths.²²

Fortunately, tailpipe emissions of criteria pollutants from the average automobile have been reduced since 1968 by more than 60% as a result of tighter standards set in the Clean Air Act. Unfortunately, lower emissions per mile have been offset largely by increased numbers of vehicles and driving miles. Further reductions in per vehicle automobile emissions undoubtedly will occur as new fuel reformulations, engine controls, and tailpipe pollution control equipment are introduced. However, using these strategies to improve ambient air quality in densely populated areas may be less effective than using emerging alternative fuels and renewables-based propulsion systems.

Environmental problems due to transportation are on the same order of magnitude as those problems resulting from power generation. Together, these two sectors account for most of the air pollution regulated under the Clean Air Act. Emissions of nitrogen oxides, hydrocarbons, fine particulate matter, and carbon monoxide from electricity generation are now below those stemming from transportation. Electrical generation surpasses transportation slightly for emissions of carbon dioxide but remains by far the largest source of sulfur dioxide, accounting for 72% of the nation's total.²³

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