Ozone is a gas that occurs both in the Earth's upper atmosphere and at ground level. Ozone can be "good" or "bad" for people’s health and for the environment, depending on its location in the atmosphere.

In the troposphere, the air closest to the Earth's surface, ground-level or "bad" ozone is a pollutant that is a significant health risk, especially for children with asthma. It also damages crops, trees and other vegetation. It is a main ingredient of urban smog.

The stratosphere, or "good" ozone layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays. This natural shield has gradually been damaged or "depleted" by man-made chemicals like chlorofluorocarbons (CFCs). Ozone depletion means more UV from the sun gets to the ground, leading to more cases of skin cancer, cataracts, and other health problems.