Ozone is a gas that occurs both in the Earth s upper atmosphere and at ground level.
Ozone occurs naturally in the Earth s upper atmosphere 10 to 30 miles above the Earth s surface where it shields us from the sun s harmful ultraviolet rays. In the Earth s lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight. Ozone pollution is a concern during the summer months when the weather conditions needed to form ground-level ozone lots of sun, hot temperatures normally occur.