Ultraviolet, or UV, light is a form of electromagnetic radiation that lies beyond the violet end of the visible light spectrum. It has a shorter wavelength, and therefore carries more energy, than visible light. UV radiation is a component of sunlight, but much of it is filtered out by the Earth’s ozone layer and only a small proportion reaches the surface. Ultraviolet light stimulates vitamin D production and is responsible for tanning; however, the higher energy levels in UV light can also damage cells. UV irradiation can be used to kill harmful microorganisms, but in humans, overexposure to UV can cause sunburn and eye problems as well as increasing the risk of skin cancer.
UV light can be classified into types A, B and C, according to wavelength, with A having the longest wavelength and C the shortest. Although UV irradiation of the Earth involves all three types, shorter wavelengths are absorbed more efficiently by the atmosphere, so that more UVA reaches the surface than UVB and all the UVC is absorbed, except at very high altitudes. Since UVB has the shorter wavelengths, it delivers more energy. This energetic form of UV light can break the bonds that hold molecules together, causing chemical changes in materials exposed to UV irradiation.
The effects of UV radiation can be both beneficial and harmful. Ultraviolet light stimulates the production of vitamin D3, which is essential to human health. Lack of exposure to sunlight can result in a deficiency, with associated health effects.
Vitamin D3 is produced naturally from a precursor compound found in the skin on exposure to UVB. Another form of the vitamin, known as D2, can be produced by UV irradiation of a compound found in plants. This has similar effects to D3 and is available as a vitamin supplement.
UVA stimulates production of the pigment melanin in the skin, resulting in tanning, although some UVB is required to bring the melanin to the surface. Tanning beds use UV lamps designed to produce mainly UVA, in order to promote tanning and minimize the risk of burning and skin cancer. Evidence suggests, however, that UVA may also contribute to skin cancer, and that there is an increased incidence of the disease in people who regularly use tanning beds. UVB is more damaging, but UVA, because of its longer wavelength, penetrates deeper into the skin. Both types contribute to aging of the skin and excessive exposure can cause cataracts in the eye.
There is concern that depletion of ozone in the upper atmosphere will allow larger amounts of UVB to reach the surface, resulting in damage to living organisms. Both land-dwelling and aquatic life may be affected. Phytoplankton, the simple marine photosynthesizing organisms that form the crucial first level of the marine food chain, can be affected by UV down to a depth of up to 66 feet (20 meters).
Due to specific concerns about the effects of UV radiation on human health, guidelines for reporting of UV levels have been published by the World Health Organization. In many areas, a UV index is used to alert the public to the likely level of UV radiation for the next day. In the USA, a UV index ranging from 1 to 15 has been developed by the Environmental Protection Agency (EPA) to indicate the risk of skin damage. An index of 2 or less is low, while a value of 11+ is extreme.
Damage to organisms results mainly from UV's effect on DNA, which absorbs radiation at UVB wavelengths. Although repair mechanisms have evolved to restore damaged DNA, the effects of UV irradiation can prevent cells from reproducing. UV irradiation can also induce cell death or abnormal growth, leading to cancer.
The damaging effects of UV irradiation can be put to good use as a means of killing or deactivating pathogenic microorganisms. As they consist of single cells, normally with little or no protection from radiation, microorganisms are especially vulnerable to damage from UV light. Pathogenic viruses and bacteria do not usually survive for long when outdoors and exposed to sunlight. An ultraviolet germicide that provides more intense UV radiation than sunlight can be used effectively for disinfection in hospitals and for sterilizing water.