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. Lawn mowers, chain saws, power boats and snowmobiles are some other sources of petroleum pollution. Hydrocarbons also evaporate from paint and protective coatings, and from some solvents used in printing and industrial processes.