Ozone is a compound composed of three atoms of oxygen. At ground level, ozone is harmful because it acts as a pollutant; in the stratosphere, ozone protects the Earth from the harmful ultraviolet rays of the sun.
Ozone makes up only three molecules per 10 million air molecules. Ozone molecules in the atmosphere form a layer that absorbs ultraviolet light. Ozone-destroying molecules such as chlorofluorocarbons bond with ozone molecules, thereby destroying them. These destructive chemicals have caused thinning of the ozone layer in certain parts of the world.
The ozone layer protects organisms on Earth from ultraviolet rays, which damage genetic material such as DNA. The Montreal Protocol on Substances that Deplete the Ozone Layer of 1987 called for the phasing out of ozone-destroying chemicals in favor of more ozone-friendly ones.