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
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
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
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.
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 1015 to 1017 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.
See L. R. Koller, Ultraviolet Radiation (2d ed. 1965).
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.
world meteorological organization. (2003). scientific assessment of ozone depletion: 2002. global ozone research and monitoring project, report no. 47. geneva: author.
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
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.
Radiation of the sun including ultraviolet A (UV-A, 320-400 nanometers) and ultraviolet B (UV-B, 280-320 nanometers). Exposure to UV-A radiation, which is utilized in tanning booths, damages dermal elastic tissue and the lens of the eye, and causes cancer in hairless mice. Exposure to UV-B induces breaks and other mutations in DNA and is associated with basal and squamous cell carcinoma as well as melanoma. The ozone layer of the earth's atmosphere provides protection from ultraviolet radiation, but this protective layer is becoming depleted due to the release of chlorofluorocarbons and other causes.