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          Cars release pollutants as the result of the fuel combustion process.  These pollutants are:

Hydrocarbons which react with nitrogen oxides in the presence of sunlight and elevated temperatures to form ground-level ozone. It can cause eye irritation, coughing, wheezing, and shortness of breath and can lead to permanent lung damage.

Nitrogen oxides (NOx) also contribute to the formation of ozone and contribute to the formation of acid rain and to water quality problems.

Carbon monoxide is a colorless, odorless, deadly gas. It reduces the flow of oxygen in the bloodstream and can impair mental functions and visual perception. In urban areas, motor vehicles are responsible for as much as 90 percent of carbon monoxide in the air.

Vehicle emission systems are designed to reduce toxic emissions.  Computers regulate the fuel and air mixtures to assure that the fuel is burning properly.  The waste gases from the burning fuel are passed through a catalytic converter that converts the waste into cleaner elements. 

Emissions safeguards are designed to last the life of the vehicle.  However, a poor running engine may feed too much gas to the cylinders, or old seals and gaskets may ad oil to the combustion process, and the result may be a clogged or cracked catalytic converter.