Ozone Depletion Some of the CFCs (chemicals containing chlorine, fluorine and carbon - chloroflorocarbons, that are particularly good as refrigerants, degreasing agents and for driving foams from aerosols are now controlled by the Montreal Protocol. This is an international agreement to phase out their use because of the damage that they cause to the Ozone layer. The ozone layer, 25,000 m above sea level, is vital to life on Earth as it filters the ultra violet rays from the sun. Ozone is a gas of molecules of three atoms of oxygen, chemically written as O 3. In 198xx a hole was discovered in the Ozone layer which led to increased UV in the Southern hemisphere and increased levels of skin cancer. The hole is seasonal but is larger each year. One of the problems is that the CFCs remain in the upper atmosphere for many years and so will continue to damage the ozone layer for years to come even if we manage to completely eliminante the use of CFCs. Increased UV radiation into the Earth's atmosphere also contributes to global warming - see next box. Ground Level Ozone Ozone in the upper atmosphere forms a protective layer which filters out dangerous UV rays from the Sun. Ground level ozone, on the other hand , is itself very damaging to human health . It is formed by complex chemical reactions from vehicle exhausts.