Ultraviolet rays and radiation

Just like visible light, infrared light, and radio waves, ultraviolet light is electromagnetic radiation. On the spectrum, ultraviolet light lies between violet light and x rays, with wavelengths ranging from four to 400 nanometers. Although it is undetectable to the naked eye, anyone who has been exposed to too much sunlight has probably noted the effects of ultraviolet light, for it is this radiation that causes tanning, sunburn, and can lead to skin cancer.

The man credited with the discovery of ultraviolet light is the German physicist Johann Ritter. Ritter had been experimenting with silver chloride, a chemical known to break down when exposed to sunlight. He found that the light at the blue end of the visible spectrum—blue, indigo, violet—was a much more efficient catalyst for this reaction. Experimenting further, he discovered that silver chloride broke down most efficiently when exposed to radiation just beyond the blues, radiation that was invisible to the eye. He called this new type of radiation ultraviolet, meaning "beyond the violet." While ultraviolet radiation in large doses is hazardous to humans, a certain amount is required by the body. As it strikes the skin, it activates the chemical processes that produce Vitamin D. In areas that lack adequate sunshine, children are sometimes plagued by rickets. In order to treat these cases, or to supplement natural light in sun-starved communities, ultraviolet lamps are often used in place of natural sources.

There are three varieties of ultraviolet lamps, each producing ultraviolet light of a different intensity. Near-ultraviolet lamps are fluorescent lights whose visible light has been blocked, releasing ultraviolet radiation just beyond the visible spectrum. These lamps are also known as black lights, and are primarily used to make fluorescent paints and dyes "glow" in the dark. This effect is often seen in entertainment, but can also be used by industry to detect flaws in machine parts.

Middle-ultraviolet lamps produce radiation of a slightly shorter wavelength. They generally employ an excited arc of mercury vapor and a specially designed glass bulb. Because middle-ultraviolet radiation is very similar to that produced by the Sun , these lamps are frequently used as sunlamps and are often found in tanning salons and greenhouses. Photochemical lamps generating middle-ultraviolet light are also used in industry, as well as by chemists to induce certain chemical reactions.

Far-ultraviolet lamps produce high-energy, short-wavelength ultraviolet light. Like middle-ultraviolet lamps, they use mercury-vapor tubes; however, far-ultraviolet radiation is easily absorbed by glass, and so the lamp's bulb must be constructed from quartz . Far-ultraviolet light has been found to destroy living organisms such as germs and bacteria; for this reason, these lamps are used to sterilize hospital air and equipment. Far-ultraviolet radiation has also been used to kill bacteria in food and milk, giving perishables a much longer shelf life.

A more passive application of ultraviolet light is in astronomy . Much of the light emitted by stars, particularly very young stars, is in the ultraviolet range. By observing the output of ultraviolet light, astronomers can determine the temperature and composition of stars and interstellar gas, as well as gain insights into the evolution of galaxies. However, most of the ultraviolet light from distant sources is unable to penetrate the Earth's atmosphere; therefore, ultraviolet observations must be made from Earth's orbit by sounding rockets, space probes, or astronomical satellites.

See also Electromagnetic spectrum; Solar illumination: Seasonal and diurnal patterns

ULTRAVIOLET RADIATION

The principal adverse health effects of sunlight are caused by the ultraviolet and visible radiation it contains. Ultraviolet radiation (UVR) comprises a spectrum of electromagnetic waves of different wavelengths, subdivided for convenience into three bands, which are measured in nanometers (nm):(1) UVA ("black light"), 315 to 400 nm; (2) UVB, 280 to 315 nm; and (3) UVC (which is germicidal), 200 to 280 nm. Visible light consists of electromagnetic waves varying in wavelength from about 400 (violet) to 700 nm (red).

None of these radiations penetrates deeply into human tissue, so that the injuries they cause are confined chiefly to the skin and eyes. Reactions of the skin to UVR are common among fair-skinned people and include sunburn, skin cancers (basal cell and squamuous cell carcinomas, and to a lesser extent melanomas), aging of the skin, solar elastoses, and solar keratoses. Injuries of the eye include photokeratitis, which may result from prolonged exposure to intense sunlight ("snow blindness"); photochemical blue-light injury of the retina, from gazing directly at the sun; cortical cataract of the lens; and uveal melanoma.

The effects of UVR result chiefly from its absorption in DNA, resulting in the cross-linkage of pyriminide nucleotides, which, in turn, may cause mutations in exposed cells. Sensitivity to UVR may be decreased by DNA repair defects, by agents that inhibit the repair enzymes, and by photosensitizing agents (such as psoralens, sulfonamides, tetracyclines, and coal tar) that increase the absorption of UVR in DNA.

To prevent injury by sunlight, excessive exposure to the sun should be avoided—especially by fair-skinned individuals—and protective clothing, UVR-screening lotions or creams, and UVR-blocking sunglasses should be used when necessary. Also, although the sun is unlikely to cause a retinal burn under normal viewing conditions since bright, continuously visible light normally elicits an aversion response that acts to protect the eye against injury, one must never gaze at the sun nor look directly at a solar eclipse.

From an environmental perspective, it is noteworthy that the protective layer of ozone in the stratosphere is gradually being depleted by chlorofluorocarbons and other air pollutants, and that every 1 percent decrease in stratosphereic ozone shield is expected to raise the UVR reaching the earth sufficiently to increase the frequency of skin cancer by 2 to 6 percent. Of potentially greater significance for human health than the projected increase in cancer rates, however, are the farreaching impacts on vegetation and crop production that may result from depletion of the ozone shield.

Arthur C. Upton

Bibliography

American Medical Association, Council on Scientific Affairs (1989). "Harmful Effects of Ultraviolet Radiation." Journal of the American Medical Association 262:380–384.

English, D. R.; Armstrong, B. K.; Kricker, A.; and Fleming, C. (1997). "Sunlight and Cancer." Cancer Causes and Control 8:271–283.

Henriksen, T.; Dahlback, A.; Larsen, S.; and Moan, J. (1990). "Ultraviolet Radiation and Skin Cancer. Effect of an Ozone Layer Depletion." Photochemical Photobiology 51:579–582.

Zabriske, N. A., and Olson, R. J. (1998). "Occupational Eye Disorders." In Environmental and Occupational Medicine, 3rd edition, ed. W. N. Rom. Philadelphia, PA: Lippincott-Raven.

Ultraviolet radiation

Just like visible light, infrared light, and radio waves, ultraviolet light is electromagnetic radiation. Ultraviolet light lies on the spectrum between violet light and X-rays . Although ultraviolet radiation is undetectable to the naked eye, anyone who has been exposed to too much sunlight has probably noted the effects of ultraviolet light. It is this form of radiation that causes tanning and sunburn, types of skin damage which can lead to skin cancer.

Ritter's Experiments

The man credited with the discovery of ultraviolet light is German physicist Johann Ritter (1776-1910). Ritter experimented with silver chloride, a chemical known to break down when exposed to sunlight. He found that the light at the blue end of the visible spectrum (blue, indigo, and violet) was a much more efficient stimulant for this reaction. Experimenting further, Ritter discovered that silver chloride broke down most efficiently when exposed to radiation just beyond the blues. He called this new type of radiation ultraviolet, meaning "beyond the violet."

Ultraviolet Radiation and the Body

While ultraviolet radiation in large doses is hazardous to humans, a certain amount is actually required by the body. As it strikes the skin, ultraviolet rays activate the chemical processes that produce vitamin D. In areas that lack adequate sunshine, children are often plagued by rickets (a disease characterized by abnormally shaped and structured bones). In order to treat these cases, or to supplement natural light in sun-starved communities, ultraviolet lamps are often used.

Three Types of Lamps

There are three varieties of ultraviolet lamps, each producing ultraviolet light of a different intensity. Near-ultraviolet lamps are fluorescent lights whose visible light has been blocked, releasing ultraviolet radiation just beyond the visible spectrum. These lamps are also known as black lights, and are primarily used to make fluorescent paints and dyes "glow" in the dark. This effect is often used for entertainment, but can also be used by industry to detect flaws in machine parts.

Middle-ultraviolet lamps produce radiation of a slightly shorter wave-length. They generally employ an excited arc of mercury vapor and a specially designed glass bulb. Because middle-ultraviolet radiation is very similar to that produced by the sun, these lamps are frequently used as sunlamps. They are often found in tanning salons and greenhouses. Photochemical lamps generating middle-ultraviolet light are also used in chemical laboratories and industrial settings to induce certain chemical reactions.

Far-ultraviolet lamps produce high-energy, short-wavelength ultraviolet light. Like middle-ultraviolet lamps, they use mercury-vapor tubes. Far-ultraviolet radiation is easily absorbed by glass, so the lamp's bulb must be constructed from quartz. Far-ultraviolet light has been found to destroy living organisms such as germs and bacteria. These lamps are often used to sterilize hospital air and equipment. Far-ultraviolet radiation has also been used to kill bacteria in food and milk, giving perishables a much longer shelf life.

ultraviolet radiation invisible electromagnetic radiation between visible violet light and X rays; it ranges in wavelength from about 400 to 4 nanometers and in frequency from about 10 ¹⁵ to 10 ¹⁷ hertz. It is a component (less than 5%) of the sun's radiation and is also produced artificially in arc lamps, e.g., in the mercury arc lamp.

The ultraviolet radiation in sunlight is divided into three bands: UVA (320–400 nanometers), which can cause skin damage and may cause melanomatous skin cancer ; UVB (280–320 nanometers), stronger radiation that increases in the summer and is a common cause of sunburn and most common skin cancer; and UVC (below 280 nanometers), the strongest and potentially most harmful form. Much UVB and most UVC radiation is absorbed by the ozone layer of the atmosphere before it can reach the earth's surface; the depletion of this layer is increasing the amount of ultraviolet radiation that can pass through it. The radiation that does pass through is largely absorbed by ordinary window glass or impurities in the air (e.g., water, dust, and smoke) or is screened by clothing.

The National Weather Service's daily UV index predicts how long it would take a light-skinned American to get a sunburn if exposed, unprotected, to the noonday sun, given the geographical location and the local weather. It ranges from 1 (about 60 minutes before the skin will burn) to a high of 10 (about 10 minutes before the skin will burn).

A small amount of sunlight is necessary for good health. Vitamin D is produced by the action of ultraviolet radiation on ergosterol, a substance present in the human skin and in some lower organisms (e.g., yeast), and treatment or prevention of rickets often includes exposure of the body to natural or artificial ultraviolet light. The radiation also kills germs; it is widely used to sterilize rooms, exposed body tissues, blood plasma, and vaccines.

Ultraviolet radiation can be detected by the fluorescence it induces in certain substances. It may also be detected by its photographic and ionizing effects. The long-wavelength, "soft" ultraviolet radiation, lying just outside the visible spectrum, is often referred to as black light; low intensity sources of this radiation are often used in mineral prospecting and in conjunction with bright-colored fluorescent pigments to produce unusual lighting effects.

Bibliography: See L. R. Koller, Ultraviolet Radiation (2d ed. 1965).

Ultraviolet Radiation

Ultraviolet (UV) radiation is a form of electromagnetic radiation that lies between visible light and x rays in its energy and wavelength. It is a component of the radiation that reaches the Earth from the sun. The broad UV band, having wavelengths between 190 nanometers (nm) and 400 nm, is conventionally divided into three parts: UV-A or near-UV (315 to 400 nm), UVB or mid-UV (280 to 315 nm), and UV-C or far-UV (190 to 280 nm). Much of the incident solar UV radiation is absorbed by gases in the earth's atmosphere and never reaches the earth's surface. This is fortunate, because UV radiation can chemically alter important biological molecules, including proteins and deoxyribonucleic acid (DNA), and thereby cause damage to living systems. The most familiar effect on humans is sunburn, which is the manifestation of UV's damage to outer skin cells. Long-term effects of excessive UV exposure include skin cancer, eye damage (cataracts), and suppression of the immune system.

Among the atmospheric gases that are the major absorbers of UV radiation is ozone (O3), which lies predominantly in the upper atmospheric region known as the stratosphere. Stratospheric ozone is particularly important in absorbing UV-B radiation. A current environmental issue concerns the depletion of stratospheric ozone (the ozone layer) by human-made chemicals such as chlorofluorocarbons (CFCs) and halons. With even small percentages of ozone depletion, more UV-B radiation reaches the surface of the earth and the harmful effects of UV increase.

see also CFCs (Chlorofluorocarbons); Halon; Ozone.

Bibliography

world meteorological organization. (2003). scientific assessment of ozone depletion: 2002. global ozone research and monitoring project, report no. 47. geneva: author.

internet resource

nasa advanced supercomputing division web site. "ultraviolet radiation." available from http://www.nas.nasa.gov/about/education/ozone/radiation.html.

united nations environment programme. (1998). "environmental effects of ozone depletion 1998 assessment." in the global change research information office web site. available from http://www.gcrio.org/ozone/toc.html.

world meteorological organization. "uv radiation page." available from http://www.srrb.noaa.gov/uv.

Christine A. Ennis

ultraviolet radiation (UV) Electromagnetic radiation having wavelengths between that of violet light and long X-rays, i.e. between 400 nanometres and 4 nm. In the range 400–300 nm the radiation is known as the near ultraviolet. In the range 300–200 nm it is known as the far ultraviolet. Below 200 nm it is known as the extreme ultraviolet or the vacuum ultraviolet, as absorption by the oxygen in the air makes the use of evacuated apparatus essential. The sun is a strong emitter of UV radiation but only the near UV reaches the surface of the earth as the ozone layer of the atmosphere absorbs all wavelengths below 290 nm. Ultraviolet radiation is classified in three ranges according to its effect on the skin: UV-A (320–400 nm), UV-B (290–320 nm), and UV-C (230–290 nm).

The longest-wavelength range, UV-A, is not harmful in normal doses and is used clinically in the treatment of certain skin complaints, such as psoriasis. It is also used to induce vitamin D formation in patients that are allergic to vitamin D preparations. UV-B causes reddening of the skin followed by pigmentation (tanning). Excessive exposure can cause severe blistering. UV-C, with the shortest wavelengths, is particularly damaging. It is thought that short-wavelength ultraviolet radiation causes skin cancer and that the risk of contracting this has been increased by the depletion of the ozone layer.

Most UV radiation for practical use is produced by various types of mercury-vapour lamps. Ordinary glass absorbs UV radiation and therefore lenses and prisms for use in the UV are made from quartz.

ultraviolet radiation (UV) Electromagnetic radiation at wavelengths between 100 and 400 nanometres (nm), lying just beyond the high-energy (violet) end of the visible-light band of the solar spectrum. Radiation at UV wavelengths comprises about 5 per cent of the total energy the Earth receives from the Sun (55 per cent is in the infrared and 40 per cent in the visible light wavebands). The UV waveband is conventionally divided into three: UVA, 315–400 nm; UVB, 280–315 nm; and UVC, 100–280 nm. All the UVC radiation and some of the UVB is absorbed by the atmosphere (mainly in the ozone layer). Prolonged exposure to UVB causes sunburn in fair-skinned humans and can lead to non-melanoma skin cancers; a link between UVB exposure and melanomas is probable, but other factors are also implicated. Certain plant cells are also damaged by UVB, but threats to phytoplankton are limited, because UV is absorbed strongly by sea water (25 per cent is absorbed in the uppermost metre of water and none penetrates below 20 m).

ultraviolet radiation (UV) (ultră-vy-ŏ-lit) n. invisible short-wavelength radiation beyond the violet end of the visible spectrum, which is emitted by the sun. UVA UV radiation in the range 310–400 nm, which is not filtered by the atmosphere and penetrates deeply into the skin, causing age-related wrinkling and drug-induced photosensitivity reactions and contributing to the development of skin cancer. UVB UV radiation in the range 280–310 nm, which is largely filtered by the atmosphere. Depending on its strength, it causes suntan and sunburn and contributes to the development of skin cancer.

ultraviolet radiation Type of electromagnetic radiation of shorter wavelength and higher frequency than visible light. Wavelengths range from four to 400nm (nanometres). Sunlight contains ultraviolet rays, most of which are filtered by the ozone layer. If the ozone layer is weakened, enough ultraviolet can reach the ground to harm living things. Excessive exposure to sunlight can cause sunburn and skin cancer in people with fair skins. Ultraviolet is used medically to sterilize equipment. See also light; radiation

ultraviolet (uv) irradiation Light of wavelength below the visible range. Wavelength for maximal germicidal action is 260 nm; it has poor penetrating power and is only of value for surface sterilization or sterilizing air and water. Also used for tenderizing and ageing of meat, curing cheese, and prevention of mould growth on the surface of bakery products.

Ultraviolet from sunlight is responsible for skin tanning, and the formation of vitamin D from 7‐dehydrocholesterol in the skin.

ultraviolet radiation The region of the electromagnetic spectrum spanning the wavelength range from the Lyman limit at 91.2 nm to 350 nm. It can be split into the shorter-wavelength far ultraviolet and the longer-wavelength near ultraviolet, the boundary between the two lying at approximately 200 nm. Overlapping slightly with the far ultraviolet is the extreme ultraviolet waveband, spanning the range 10–100 nm.

ultraviolet radiation Electromagnetic radiation which has a wavelength between 0.5 nm and 400 nm, located between the visible and X-ray regions of the electromagnetic spectrum. Near ultraviolet occurs at wavelengths between 400 nm and 300 nm, middle ultraviolet between 300 nm and 200 nm, and extreme ultraviolet between 200 nm and 150 nm.