Ultraviolet radiation is made up of three types of rays -- ultraviolet A, ultraviolet B, and ultraviolet C. Although ultraviolet C is the most dangerous type of ultraviolet light in terms of its potential to harm life on earth, it cannot penetrate earth's protective ozone layer. Therefore, it poses no threat to human, animal or plant life on earth.
Ultraviolet A and ultraviolet B, on the other hand, do penetrate the ozone layer in attenuated form and reach the surface of the planet. Because ultraviolet A is weaker than ultraviolet B, scientists long considered ultraviolet B the sole culprit in causing skin cancer in persons with a history of sunburn and repeated overexposure to ultraviolet radiation. Recent research, however, has also implicated ultraviolet A as a possible cause of skin cancer. Ultraviolet B is more likely than ultraviolet A to cause sunburn, but ultraviolet A passes further into the skin. Ultraviolet B can cause melanoma and other types of skin cancer. Ultraviolet A may add to skin damage that can lead to cancer. For this reason, skin and cancer specialists recommend that people use sunscreens that block both kinds of radiation -- ultraviolet A and B.
In addition to natural light from the sun, artificial light from tanning lamps contains ultraviolet A and ultraviolet B. Electric arc lamps can also generate ultraviolet light to heat furnaces for melting and to enable motion-picture projectors to show movies.