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HOW THE OZONE LAYER IS ATTACKED

The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) leak from equipment (1). Winds efficiently mix the troposphere and evenly distribute the gases. CFCs are extremely stable, and do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere, about 10 kilometres above the Earth's surface (2).

 Strong ultra-violet (UV) light breaks apart the ODS molecule. CFCs release chlorine atoms, and halons release bromine atoms (3). It is these atoms that actually destroy ozone. It is estimated that one chlorine atom can destroy over 100,000 ozone atoms before finally being removed from the stratosphere (4).

Ozone is constantly being produced and destroyed in a natural cycle. However, the overall amount of ozone is essentially stable. This balance can be thought of as a stream's depth at a particular location. Although individual water molecules are moving past the observer, the total depth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. The situation began to change several decades ago.

 Large increases in stratospheric chlorine and bromine, have upset that balance. In effect, they have added a siphon downstream, removing ozone faster than natural ozone creation reactions can keep up. Therefore, ozone levels fall.

 Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the surface. The more depletion, the larger the increase in incoming UVB (5). UVB has been linked to skin cancer, cataracts, damage to materials like plastics, and harm to certain crops and marine organisms. Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem (6). 

Source:  EPA's Stratospheric Protection Division