The layer of the Earth's atmosphere that surrounds us is called the troposphere. The stratosphere, the next higher layer, extends about 10-50 kilometers above the Earth's surface. Stratospheric ozone is a naturally occurring gas that filters the sun's ultraviolet (UV) radiation. A diminished ozone layer allows more radiation to reach the Earth's surface.Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction. Although natural phenomena can cause temporary ozone loss, chlorine and bromine released from man-made compounds such as CFCs are now accepted as the main cause of this depletion.

Ozone depletion is caused by the reaction of chlorine with atoms of ozone. The major contributors to the presence of chlorine in the stratosphere are chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs). These substances are mainly used in refrigerants, insulating foams, solvents, and aerosol sprays. Although CFCs and HCFCs are heavier than air, they eventually reach the stratosphere though the journey can take up to five years. This is because there exist no known processes that react with and destroy CFCs. CFCs are one of the most stable compounds invented by man. They simply do not break down in the lower atmosphere and they can remain in the atmosphere from 20 to 120 years or more. It is this stability that makes them extremely potent at ozone layer depletion.

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