Ozone Layer
Ozone Layer, a region of the atmosphere from 19 to 48 km (12 to 30 mi) above the earth's surface. Ozone concentrations of up to 10 parts per million occur in the ozone layer. 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 compounds 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 the ozone layer of the atmosphere protects life on earth from the full force of the sun's cancer-causing ultraviolet radiation, it is critically important. Thus, scientists were concerned when they discovered in the 1970s that chemicals called chlorofluorocarbons, or CFCs (see Fluorine )—long used as refrigerants and as aerosol spray propellants—posed a possible threat to the ozone layer. Released into the atmosphere, these chlorine-containing chem