Ultraviolet (UV) radiation is invisible electromagnetic radiation with wavelengths that are shorter than the visible light spectrum and longer than x rays.
Ultraviolet radiation comes primarily from sunlight. Very small amounts are emitted from black lights, bug zappers, specialized lamps and some medical and scientific equipment. The UV radiation spectrum ranges from wavelengths of 400 nanometers (nm) to 10 nm. The shorter the wavelength, the higher the energy and the greater the potential to damage the body. Three types of UV radiation are of primary concern to human health: ultraviolet A (UV-A, 320–400 nm), ultraviolet B (UV-B, 280–320 nm), and ultraviolet C (UV-C, 280–100 nm).
About 10% of radiation emitted by the sun is in the UV range. Another 40% is visible light and 50% is infrared light that we perceive as heat. Ozone, a gas in Earth's atmosphere, blocks all but about 3% of the UV radiation from the sun. However, it blocks some wavelengths more effectively than others.
UV-A has the longest wavelength and the least amount of energy. It pass easily through Earth's atmosphere, and about 95% of what the sun emits reaches Earth. A great deal of UV-A radiation also passes through ordinary window glass. UV-B has a shorter wavelength and more energy. Its energy allows it to react with some ozone molecules. When the ozone molecules break apart, they absorb the UV-B energy, so only some of the UV-B radiation reaches the earth. UV-C has a shorter wavelength and more energy than UV-B. All the UV-C radiation reacts with ozone in the atmosphere. Its energy is absorbed by the ozone molecules as they split apart and no UV-C radiation from the sun reaches the surface of the earth. A small amount of UV-C radiation is generated through industrial activities such as welding. UV-C can be hazardous. Workers who may be exposed to humanproduced UV-C must wear protective clothing.
Exposure to UV radiation is both helpful and harmful. The body makes a form of vitamin D when the skin is exposed to sunlight, and more specifically to UV-B. Vitamin D is a fat-soluble steroid compound that the body needs to remain healthy. Its main role is to regulate amount of calcium circulating in the blood. Calcium is a mineral acquired through diet. Vitamin D helps regulate the absorption of calcium from the small intestine. Too little vitamin D can cause weak, brittle, deformed bones. UV-B at a wavelength of around 310 nm also is used to treat psoriasis, eczema, vitiligo, and other skin conditions. In addition, UV radiation can safely be used to sterilize food, water, and in laboratories and hospitals to kill or break down the DNA of undesirable microorganisms.
However, exposure to UV-B, either from sunlight or a tanning bed, may cause sunburn, basal and squamous cell skin cancer, or malignant melanoma. UVB directly damages DNA, the genetic material, thus causing mutations. UV-A penetrates the skin and does not cause sunburn. Because of its lower energy level, it does not directly damage DNA. Nevertheless, it contributes to the development of skin cancer, especially potentially fatal melanoma, by stimulating chemical reactions that release free radicals. In April 2011, the International Agency for Research on Cancer of the World Health Organization classified all ultraviolet radiation as a carcinogen in humans.
Individuals, especially those with fair skin, must avoid excessive exposure to the sun. Sunscreens contain substances that block UV-B. The degree of blocking power is indicated by the SPF number. The higher the number, the greater the protection against UV-B radiation. Some sunscreens contain compounds such as titanium dioxide, zinc oxide, and avobenzone that block UV-A. Europe and Japan have rating systems in place to allow consumers to determine the degree of protection against UV-A each sunscreen offers. The Food and Drug Administration (FDA) regulates labeling of sunscreen in the United States. A rating system for UV-A is under consideration. However, some sunscreen products in the United States already indicate if they protect against both UV-B and UV-A.
Prevention must also occur on a global level. The ozone layer of Earth's atmosphere provides protection from ultraviolet radiation, but this protective layer has been depleted largely due to the reactions of chlorofluorocarbons (CFCs) and other ozone-depleting substances in the upper atmosphere. By 2010, there was evidence that adherence to the Montreal Protocol on Substances that Deplete the Ozone Layer and other international agreements might allow substantial ozone layer recovery by 2060. However, although the use of ozone-depleting compounds has declined due to bans on these substances, as of 2008, measurements reported by the National Oceanic and Atmospheric Administration (NOAA) indicated that the Antarctic ozone hole hit its maximum in 2006. The situation remains under study because other factors also contribute to the size of the cyclic Antarctic ozone hole.
See also Melanoma ; Ozone ; Radiation ; Skin cancer .
Stille, Darlene. Invisible Exposure: The Science of Ultraviolet Rays. Mankato, MN: Compass Point Books, 2010.
Allen, Jeannie. “Radiation—How It Affects Life on Earth.” NASA. http://earthobservatory.nasa.gov/Features/UVB (accessed October 24, 2012).
Environmental Protection Agency. “UV Radiaton.” http://www.epa.gov/sunwise/doc/uvradiation.html (accessed October 24, 2012).
World Health Organization. “Ultraviolet Radiation and Health.” http://www.who.int/uv/uv_and_health/en/index.html (accessed October 23, 2012).
American Academy of Dermatology, PO Box 4014, Schaumburg, IL, 60168-4014, (866) 503 7546, Fax: (847) 240 1859, MRC@aad.org, http://www.aad.org .
Environmental Protection Agency, Ariel Rios Bldg., 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 272-0167; TTY (202) 272-0165, http://www.epa.gov .
World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, 22 41 791 21 11, Fax: 22 41 791 31 11, email@example.com, http://www.who.int .
Tish Davidson, AM