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. The ozone layer is gradually being destroyed by human activity.

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) but the amounts balance each other out so there is no net increase or decrease due to natural processes. In 1970 Paul Crutzen showed that naturally occurring oxides of nitrogen can catalytically destroy ozone. In 1974 F. Sherwood Rowland and Mario Molina demonstrated that chlorofluorcarbons (CFCs) could also destroy ozone. In 1995 all three were jointly awarded the Nobel prize for chemistry.

The CFCs that were observed as being damaging included Freon 11 (CFCl₃) and Freon 12 (CF₂Cl...