According to the EPA, in the U.S., 75% of Carbon Monoxide emissions come from automobiles.
You see it every time that smoke billows from your car's exhaust pipe, so there's no denying that vehicles are major contributors to air pollution. Air pollution refers to the presence of foreign substances in the air that doesn't belong there or excessive amounts of certain impurities that wouldn't harm us otherwise. When cars burn gasoline, they emit pollutants. Gasoline fumes escape into the air even when we pump gasoline into our fuel tanks.
Four major pollutants come from cars:
A car emits carbon monoxide when the carbon in fuel doesn't burn completely.
A car's exhaust emits hydrocarbons, a toxic compound of hydrogen and carbon.
When fuel burns, nitrogen and oxygen react with each other and form nitrogen oxides (NOx).
Particulate matter -- small particles of foreign substances -- in the air contribute to atmospheric haze and can damage people's lungs.
Pollutants from cars contribute to various types of air pollution. When hydrocarbons and NOx combine in sunlight, they produce ozone. High in the atmosphere, ozone protects us from the sun's ultraviolet rays. When holes in the atmosphere's ozone layer allow ozone to come closer to Earth, it contributes to smog and causes respiratory problems.
Air pollutants emitted from cars are believed to cause cancer and contribute to such problems as asthma, heart disease, birth defects, and eye irritation.