The Ozone Layer is a region of the atmosphere from 19 to 48 km to above the earth's surface. The ozone forms there by the action of sunlight on oxygen. This action has been taking place for many millions of years, but naturally occurring nitrogen gas in the atmosphere apparently have kept the ozone concentration at a fairly stable level. Concentrations this great at ground level are dangerous to breathe and can damage the lungs. However, because …
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… HCFCs, which cause less damage to the ozone layer than CFCs do, are being used as substitutes for CFCs on an interim basis, until 2020 in developed countries and until 2016 in developing countries. To monitor ozone depletion on a global level, in 1991 NASA launched the 7-ton Upper Atmosphere Research Satellite. Orbiting earth at an altitude of 600 km, the spacecraft measures ozone variations at different altitudes and is providing the first complete picture of upper atmosphere.