Levels of many outdoor air pollutants decreased substantially after the passage of the Clean Air Act of 1970; however, levels of ozone, carbon monoxide, and particulate matter are still high enough to present hazards to children. Failure to meet the federal standards for these pollutants was a major force driving the adoption of the revised Clean Air Act of 1990. In addition, recent research indicates that acidic aerosols, for which there are no health-based standards, may be associated with adverse respiratory effects.

As an ambient air pollutant, ozone is formed by the action of sunlight on nitrogen oxides and reactive hydrocarbons (both of which are emitted by motor vehicles and industrial sources). Ozone levels therefore tend to be highest on warm, sunny days, which are conducive to outdoor activities. In many areas ozone concentrations peak in the midafternoon, when children are likely to be playing outside. It is important to distinguish ground-level ozone air pollution from stratospheric ozone depletion by chlorofluorocarbons. These issues are unrelated. Carbon monoxide, a product of incomplete combustion, is emitted mainly from cars and other mobile sources. Airborne particulate matter is a variable and complex mixture of natural materials and substances released from numerous industries, motor vehicles, residential wood burning, construction and demolition, and other sources. Acidic aerosols are traceable mainly to combustion of sulfur-containing fossil fuels and to reactions of photochemical free radicals with nitrogen dioxide.

Exposure to ambient air pollution in North America has been clearly associated with acute and subacute effects in epidemiologic investigations and in controlled exposure studies in environmental chambers.

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