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.
• Basic Information about Ground Level Ozone
• AirNow: Current ground level ozone levels where you live
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.
• More information on what’s happening to the ozone layer and regulations about CFCs and substitutes
• SunWise: Information on protecting yourself from the sun
EPA’s Clean Air Ozone Rules target ground level ozone. On April 15, 2004, EPA fulfilled its legal obligation under the Clean Air Act to issue final designations for all areas of the country stating that they either meet or do not meet the more protective 8-hour ozone standards. EPA also issued a classification rule for nonattainment areas -- areas that do not meet the standards. The rule classifies the seriousness of the problem and indicates the required actions that areas must take. More information
For an overview of both ozone issues, see Good Up High, Bad Nearby.