In the lower eight miles of air - called the troposphere - ozone is a health problem: a common, nasty ingredient of city smog that irritates the eyes and lungs.

Sunlight mixed with man-made pollution - such as nitrogen oxides and organic compounds - changes "good" two-atom oxygen into the "bad" three atom oxygen compound called ozone. If ozone in the air reaches 120 parts per billion (120 out of every one billion air molecules) in any community, it becomes a serious health risk.

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