As well as providing protection against UV radiation, ozone also acts as a greenhouse gas. Ozone occurs in the atmosphere at altitudes of up to 50 kilometres. The maximum concentration is present at about 25 kilometres - this is the ozone layer, which protects us against harmful UV radiations.

It is because of this protective effect that the thinning of the ozone layer ("the ozone hole") poses a serious problem for the human race. Once the link between the ozone hole and the emission of chlorofluorocarbons (CFC) into the atmosphere had been proven, CFCs were quickly banned at international level (by the Montreal Protocol of 1987). The ban is showing some effects: in 2000, the concentration of CFCs in the upper atmosphere began to decrease.