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 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

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

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.