Stratospheric ozone forms a protective layer that blocks out UV-B's destructive effects.
In recent years scientists have discovered troubling changes in this protective layer. In 1985, researchers reported that for several years an advancing depletion in ozone levels was occurring over the Antarctic. The Antarctic's ozone hole is a region in which about one half of the ozone is depleted during September and October each year. More recently, scientists found lowered ozone levels over the globe as a whole. In April 1991, the Environmental Protection Agency announced that ozone levels in the Northern Hemisphere shrank as much as 5 percent over the United States in the past 10 years.