Good ozone occurs naturally in the Earth's Stratosphere (upper atmosphere) 10 to 30 miles above the Earth's surface-where it shields us from the sun's harmful ultraviolet rays called UVB band. 90% of the Earth's ozone is in the stratosphere and is referred to as “Ozone Layer.
Bad ozone occurs in the Earth's lower atmosphere, near ground level, when pollutants emitted by cars, power plants, industrial boilers, 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 and hot temperatures) naturally occur.