Since the mid-1970s scientists have been concerned about the harmful effects of certain chlorofluorocarbons (CFCs) on the ozone layer. These CFC compounds are relatively inert and nontoxic, and humans found a use for them as cooling agents in refrigerators and air conditioning systems amongst other things. Because they are so inert, when CFCs become discharged into the atmosphere they diffuse and do not decompose straight away - not until they are hit by shortwave UV radiation. This occurs in the ozone layer.

When a CFC is hit by UV radiation, it loses its chlorine atom. This chlorine atom acts as a catalyst. It is able to steal one oxygen atom away from an ozone molecule, leaving an oxygen molecule and chlorine oxide. Chlorine oxide can then react with a single oxygen atom to form an oxygen molecule and a chlorine atom. This cycle means the chlorine atom is free to break up another ozone molecule.

One chlorine atom can destroy up to 100,000 ozone molecules before it is removed by some other reaction. This can devastating for the ozone layer. It disrupts the delicate flux and causes ozone to be destroyed faster than it is created.

Polar stratospheric clouds also catalyse ozone depletion by active chlorine.