Ozone is a highly toxic invisible gas. While ozone at stratospheric levels provides crucial protection from harmful ultraviolet radiation, ozone at ground level is toxic. At levels routinely encountered in most American cities, ozone, or smog, oxidizes pulmonary tissue, burning holes through the lung's cell walls, allowing cellular fluids to seep out. Over time, normal ciliated cells are destroyed by ozone and replaced by abnormally squat, thick-walled, squamous cells, causing lungs to stiffen and decreasing the ability to breathe normally.

Prolonged exposure to ozone increases susceptibility to bacterial infections, possibly because ciliated cells that normally expel foreign particles and organisms have been killed and replaced by thicker, stiffer, non-ciliated cells.²⁴ Scars and lesions form in the airways.²⁵ At ozone levels that prevail through much of the year in California and other warm-weather cities, the breathing of many healthy, non-smoking young men who exercise becomes rapid, shallow and painful.²⁶ As ozone levels rise, so do hospital admissions and emergency department visits.²⁷ In New Jersey, emergency room visits for asthma increased 28 percent at ozone concentrations half the federal standard.

Every city and metropolitan area in the United States is plagued by high levels of ozone. In 1989, the exposed population was 67 million, but in warmer years like 1988, this can more than double to 135 million.²⁸ Some ozone is formed naturally, but the reaction between hydrocarbons and oxides of nitrogen prevents its destruction. Ozone can be reduced by curbing emissions of oxides of nitrogen (NO_x) from powerplants, cars and trucks; and reducing hydrocarbons, principally from gasoline and industrial solvents.

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