Check out this Nasa's Earth Observatory website about Ozone Hole and its recovery.
An excerpt:
Ozone is a highly reactive colorless gas that contains three oxygen atoms. Near the surface of the Earth, ozone is hazardous to human health, causing problems like lung irritation, but high in the atmosphere, ozone acts as a crucial shield that absorbs harmful ultraviolet radiation from the Sun.
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17116
