The ozone in the ozone layer is being broken down by chlorine atoms from chlorofluorocarbon (CFC) molecules and bromine atoms from halons. CFCs and halons are produced by humans used in many applications such as refrigerants, anaesthetics, aerosols, fire-fighting equipment and the manufacture of materials such as stryrofoam. They were thought to be completely safe, chemically inert, and environmentally neutral. However it was soon found that they were not so ideal when they reached the upper atmosphere.
When CFCs reach the upper atmosphere they are first degraded by the very high energy of UV (ultra-violet) radiation. Degradation of CFC leaves a free chlorine atom. The basic cause of ozone layer depletion is that this chlorine atom then breaks up ozone molecules. Ozone then disappears. The chlorine atoms are recreated in subsequent reactions. One chlorine atom can destroy 100,000 ozone molecules.