is a colorless, odorless, poisonous gas, formed by the incomplete combustion of any carbonaceous material (e.g., gasoline, wood, coal). CO is the most widely distributed and most commonly occurring air pollutant, with motor vehicles being the primary source of man-made emissions, although emission controls are reducing the automobile’s contribution. It is estimated that more than 90% of atmospheric CO comes from natural sources, such as decaying organic matter.

Also see catalytic converter, emissions (automotive), pollutants.