UV radiation: Ultraviolet radiation. Invisible rays that are part of the energy that comes from the sun, can burn the skin, and cause skin cancer. UV radiation is made up of three types of rays -- ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC).
UVC is the most dangerous type of ultraviolet light but cannot penetrate earth's protective ozone layer. Therefore, it poses no threat to human, animal or plant life on earth. UVA and UVB, on the other hand, do penetrate the ozone layer in attenuated form and reach the surface of the planet. UVA is weaker than UVB but passes further into the skin than UVB. It is now generally accepted that both UVA and UVB cause skin cancer, including melanoma. For this reason, sunscreens are recommended that block both kinds of radiation -- UVA and UVB.
In addition to natural light from the sun, artificial light from tanning lamps contains UVA and UVB. Electric arc lamps can also generate ultraviolet light to heat furnaces for melting and to enable motion-picture projectors to show movies.
Though ultraviolet light can damage health, it can also maintain or improve health. When ultraviolet light strikes human skin, it triggers the production of vitamin D, which promotes the growth of bones and teeth.