Levels of many outdoor air pollutants decreased substantially after the passage of the Clean Air Act of 1970; however, ozone, carbon monoxide, and particulate matter are still high enough to present hazards to children.

Epidemiologic studies undertaken in a variety of locations indicate a relationship between outdoor air pollution and adverse respiratory effects in children. The pollutants most frequently implicated in these studies have been respirable particles (notably acidic sulfates) and ozone.

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).

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