Ozone exists at all levels of the atmosphere. It is slightly concentrated at upper levels. Ozone is created in this zone by a natural process, a reaction between sunlight and stratospheric oxygen. Levels fluctuate naturally, depending on the output of the sun and its angle of intensity. When the sun returns the following spring, the level rebuilds.
Ozone does absorb some ultraviolet radiation, and excessive ultraviolet radiation can be harmful.
The facts don't support any conclusions regarding whether ozone is increasing or diminishing over the long run, however, or whether man is even more than just a small part of the cause. They return again each spring with the sunlight. Ozone levels fluctuate most dramatically over the Antarctic but CFC's are primarily produced in the Northern Hemisphere. This is not supposed to be true of ground-borne ozone, however, which is one of the primary pollutants produced by man. We do not have any data on ozone levels over long periods of time.