The microscopic particles making up diesel exhaust particulate matter are less than one-fifth the thickness of a human hair. They are small enough to penetrate deep into the lungs, where they can contribute to respiratory disease. Gasoline and diesel exhausts contain nitrogen oxides, carbon monoxide and sulfur dioxide. Nitrogen oxides can damage lung tissue, lower the body's resistance to respiratory infection and worsen chronic lung diseases such as asthma. As mentioned earlier, nitrogen oxides react in the atmosphere with hydrocarbons to form ozone, the major component of urban smog.
Ozone is a strong irritant to the eyes and respiratory tract.
It can make respiratory problems worse for people who already have asthma and other respiratory diseases. Ozone also hurts the lungs of healthy people who exercise outdoors when ozone levels are high. Carbon monoxide is a colorless, odorless gas that limits the blood's ability to transport oxygen to body tissues. Its presence in the body places a strain on people who already have cardiac or respiratory diseases, as well as pregnant women and the elderly. Another concern about gasoline is methyl tertiary butyl ether, or MTBE, which originally was used in gasoline in small amounts to reduce engine knocking.