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Ozone depletion describes two distinct, but related observations:
* a slow, steady decline of about 4 percent per decade in the total amount of ozone in Earth's stratosphere since around 1980; and * a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period.
Ozone layer thinning is mainly caused by the CFC family, (Chlorofluorocarbons), commonly known as freons. These compounds are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFC's and halons increased.