Ozone occurs in two layers of the atmosphere. The layer surrounding the earth's surface is the troposphere. It is a key ingredient of urban smog. The troposphere extends to a level about 10 miles up, where it meets the second layer, the stratosphere.
Ozone occurs naturally in the stratosphere and is produced and destroyed at a constant rate.
These ozone depleting substances degrade slowly and can remain intact for many years as they move through the troposphere until they reach the stratosphere. One chlorine or bromine molecule can destroy 100,000 ozone molecules, causing ozone to disappear much faster than nature can replace it. It can take years for ozone depleting chemicals to reach the stratosphere, and even though we have reduced or eliminated the use of many CFCs, their impact from years past is just starting to affect the ozone layer. Substances released into the air today will contribute to ozone destruction well into the future. Satellite observations indicate a world-wide thinning of the protective ozone layer.