Ultraviolet, or UV, (U-V) radiation is an invisible wavelength of light whose major source is the sun. There are several categories of UV light, including UVA (U-V-A), UVB (U-V-B), UVC (U- V-C), and Vacuum UV. Of these, only UVA and UVB reach the earth from the sun, with the other types being absorbed by the atmosphere. UV radiation is a concern because excessive amounts can harm your skin and eyes. In addition to the sun's natural radiation, UV light is also generated by many artificial sources, including black lights, mercury vapor street lamps, welding torches, and certain lasers. Special devices called germicidal lamps emit UVC light to kill bacteria. Phototherapy lamps that are used to treat skin diseases give off primarily UVA light. Tanning beds are a potent source, providing three times the sun's UVA radiation during an average session. Incandescent bulbs and halogen lamps emit low levels of UVA. Fluorescent bulbs, however, give off traces of both UVA and UVB light. While some researchers have speculated that fluorescent lights could tire the eyes or age the skin, the long-term effects of such lights are unknown.