False color images showing ozone depletion from October 1979 to October 1990. U.S. National Aeronautics and Space Administration (NASA).

The ozone layer is a part of the atmosphere between 18.6 mi and 55.8 mi (30 and 90 km) above the ground. The ozone present is responsible for blocking potentially harmful ultraviolet radiation reaching the surface of the earth. During the last twenty years, evidence has accumulated that human activity may be the cause of a generalized depletion of the ozone layer. This phenomena is global and distinct from the natural factors that induce annual ozone layer hole formation over Antarctica.

Ozone is constantly created and destroyed in natural processes (manufactured by the action of lightning on oxygen and destroyed by the action of ultraviolet radiation), however the amounts normally balance each other out so there is no net increase or decrease due to natural processes. In 1970, Paul Crutzen showed...