The Ozone-oxygen cycle reference article from the English Wikipedia on 24-Jul-2004
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Ozone-oxygen cycle

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When Ozone in the Earth's upper atmosphere is hit with solar radiation, it undergoes a chemical reaction:

O3 + radiation ---> O2 + O.

Within a fraction of a second, another reaction occurs.

O2 + O ----> O3 + heat.

These reactions lessen the severity of solar radiation on the Earth's surface.

Unfortunately, CFCs (Chlorofluorocarbons) tend to react with O3 molecules, stripping of the extra oxygen molecule and leaving O2.