High ozone concentrations irritate nasal, throat and bronchial tissues. Ozone attacks certain components of the body's defense system, raising concerns about the effects of ozone exposure on the human immune system. High concentrations of ozone can also harm forests, thereby altering wildlife habitats, lowering crop yields, and damaging materials such as rubber, plastics, synthetic fibers, dyes and paints. This occurs because hydrocarbons, nitrogen oxides and ozone itself are carried by the wind many hundreds of miles from their city origins, polluting rural areas. In large urban areas, ozone mixes with other pollutants to create the air pollution condition known as smog.
Smog reduces visibility and irritates and inflames eye tissues.
Unusually hot and dry weather fosters smog production. Motor vehicles produce both nitrogen oxides and hydrocarbons. The nitrogen oxides are created when fuel is burned and hydrocarbons are emitted when fuels fail to burn completely and when gas tanks are filled. In 1993, there were 189.4 million vehicles registered nationwide, compared with 181.3 million in 1988, and 142.1 million in 1977. The number of trucks alone mushroomed by 40 percent between 1977 and 1988.