vehicles generate three major pollutants: hydrocarbons, nitrogen oxides
(NOx), and carbon monoxide (CO). Hydrocarbons react with NOx
in the presence of sunlight to form ground-level ozone (smog). NOx
contribute to the formation of acid rain, although sulfur dioxide (SO2)
is the primary contributor. Acid rain increases the level of pollutants in
lakes, rivers, bays, and other waterways. CO, a colorless, odorless gas,
can impair mental functions and visual perception and is deadly in high
concentrations. Motor vehicles also emit large quantities of carbon
dioxide (CO2), which has the potential to trap Earth's heat and
contribute to global warming.
vehicle emission control devices have dramatically reduced pollutant
emissions per vehicle during the past 20 years. But the number of cars and
trucks on the road, and the number of miles they are driven, have doubled.
Vehicles are now driven more than a trillion miles each year in the United
States. This growth in vehicle travel is offsetting the progress achieved
through improved vehicle emission controls. Cars and trucks release
millions of tons of pollutants into the air each year.