Ultraviolet radiation is that portion of the electromagnetic spectrum between visible light (about 400 nm) and the lower limit of ionizing radiation (about 100 nm). The energy of ultraviolet radiation photons increases as the wavelength decreases. The ultraviolet spectrum is divided into the following three bands: from 315 to 400 nm, it is called UV-A; from 280 to 315 nm, it is designated UV-B; and from 100 to 280 nm, it is referred to as UV-C.1
Natural sunlight includes biologically significant amounts of energy in the UV-A and UV-B bands. The upper atmosphere filters out the UV-C radiation, although there is concern about UV-C exposure in areas where the ozone layer is absent. Employees who work in the natural environment incur the greatest occupational exposure to ultraviolet radiation. Examples of such occupations include farmers and other agricultural and forestry workers, fishermen, outdoor construction workers, landscapers, and lifeguards.
Altitude, latitude, time of year, time of day, and the thickness of the ozone layer affect UV strength. Levels are higher at higher altitudes, closer to the equator, and during summer. For latitudes between the Tropic of Cancer and the Tropic of Capricorn, levels are moderate to extreme year-round. UV strength is also increased between 10 a.m. and 4 p.m. Levels are also higher where the ozone layer is thinner. While exposure is highest under ...