http://www.epa.gov/sunwise/kids/kids_uvindex.html
U.S. Environmental Protection Agency
The ozone layer shields the Earth from harmful UV radiation. Ozone depletion, weather, and the seasons cause different amounts of UV radiation to reach the Earth. The National Weather Service (NWS) and EPA made the UV Index. You'll learn more about the UV Index and the importance of taking precautions while you enjoy the sun.