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The ozone layer refers to a region of Earth's stratosphere that absorbs most of the Sun's Ultraviolet radiation. It contains high concentration of Ozone (O3) relative to other parts of the atmosphere, although it is still very small relative to other gases in the stratosphere. Depletion of the Ozone layer in Earth's atmosphere is a relatively new phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of Ozone in Earth's stratosphere. Scientific evidence indicates that stratospheric Ozone is being destroyed by a group of manufactured chemicals, containing chlorine and/or bromine. These chemicals are called "Ozone-Depleting Substances" (ODS). A Chlorofluorocarbon or CFC is basically any of the halocarbon organic compounds consisting of carbon, hydrogen, chlorine and fluorine. Many CFCs have been widely used as refrigerants, in air conditioning, as propellants (in aerosol applications) and as solvents. CFCs are considered to be the main ODS for a specific reason. There are no removal processes or sinks for CFCs in the lower part of the Earth's atmosphere or Troposphere. As a result they are transported up into the stratosphere, where they are broken down by the Ultraviolet radiation from Sun, releasing free chlorine atoms that catalyze the decomposition of Ozone into molecular oxygen, thus depleting the Ozone layer.