A pollutant that occurs in two layers of the atmosphere. In the stratosphere, ozone protects the earth from harmful ultraviolet rays. At ground-level, ozone is an air pollutant that can be harmful to human health and the environment. It is the principal component of “smog” and is produced from the action of sunlight on air contaminants from combustion sources including automobile exhausts. Ozone levels are most likely to be elevated on hot, sunny afternoons.