Environmental effect

According to the EPA, two-thirds of CO emissions and nearly half then  ozone-forming emissions come from mobile sources, such as motor vehicles. Typically, the older the car, the more pollution it emits. The EPA has found that 30 percent of five-year-old cars emit far more than they were designed to emit, contributing to air pollution more than the other 70 percent of vehicles of a similar age. Fifty-five percent of seven-year-old cars are "gross emitters." Overall, 10 to 30 percent of the vehicles on the road create the majority of the pollution from cars and trucks.

Mobile Source Contribution to Air Pollutant Inventories — 1995

* Excludes natural sources, fugitive dust, and smoke from controlled burning and forest fires. Transportation sources represent about 33 percent of total carbon emissions.