Ultraviolet rays


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ultraviolet

 [ul″trah-vi´o-let]
denoting electromagnetic radiation of wavelength shorter than that of the violet end of the spectrum, having wavelengths of 4–400 nanometers.
ultraviolet A (UVA) ultraviolet radiation with wavelengths between 320 and 400 nm, comprising over 99 per cent of such radiation that reaches the surface of the earth. Ultraviolet A enhances the harmful effects of ultraviolet B radiation and is also responsible for some photosensitivity reactions; it is used therapeutically in the treatment of a variety of skin disorders.
ultraviolet B (UVB) ultraviolet radiation with wavelengths between 290 and 320 nm, comprising less than 1 per cent of the ultraviolet radiation that reaches the earth's surface. Ultraviolet B causes sunburn and a number of damaging photochemical changes within cells, including damage to DNA, leading to premature aging of the skin, premalignant and malignant changes, and a variety of photosensitivity reactions; it is also used therapeutically for treatment of skin disorders.
ultraviolet C (UVC) ultraviolet radiation with wavelengths between 200 and 290 nm; all of this type of radiation is filtered out by the ozone layer so that none reaches the earth's surface. Ultraviolet C is germicidal and is also used in ultraviolet phototherapy.
ultraviolet rays electromagnetic radiation beyond the violet end of the visible spectrum; they are not visible to humans. They are produced by the sun but are absorbed to a large extent by particles of dust and smoke in the earth's atmosphere. They are also produced by the so-called sun lamps. They can produce sunburn and affect skin pigmentation, causing tanning. When they strike the skin surface they transform provitamin D, secreted by the glands of the skin, into vitamin D, which is then absorbed into the body. Because ultraviolet rays are capable of killing bacteria and other microorganisms, they are sometimes used to sterilize objects in specially designed cabinets, or to sterilize the air in operating rooms and other areas where destruction of bacteria is necessary.
ultraviolet therapy the employment of ultraviolet radiation in the treatment of diseases, particularly those affecting the skin. See also PUVA therapy and photochemotherapy. Among the diseases that respond to this form of therapy are acne vulgaris, psoriasis, and external ulcers.

Dosage. The dosage unit of ultraviolet radiation is expressed as minimal erythema dose (MED). Because of varying degrees of skin thickness and pigmentation, human skin varies widely in its sensitivity to ultraviolet radiation. The MED refers to the amount of radiation that will produce, within a few hours, minimal erythema (redness caused by engorgement of capillaries) in the average Caucasian skin. Dosage for individual patients is prescribed according to probable sensitivity as determined by that individual's skin type as compared to average sensitivity.
Degrees of Erythema. Minimal erythema is a first degree erythema and usually is produced after about 15 seconds of exposure to a high-pressure mercury arc in a quartz burner placed at a distance of 75 cm (30 in) from the skin. A second degree erythema results from a dose of about 2.5 MED; its effects become apparent about 4 to 6 hours after application and are followed by slight peeling of the skin. A third degree erythema is produced by about 5 MED; it may become apparent within 2 hours after application and is accompanied by edema followed by marked desquamation. A fourth degree erythema is produced by about 10 MED and is characterized by blistering.
Precautions. Ultraviolet therapy is safe only in the hands of a skilled and knowledgeable therapist. Areas of “thin skin” that may be burned more readily than that receiving treatment must be protected by wet towels or dressings. The eye is highly sensitive to ultraviolet radiation; therefore some form of protection, such as goggles, compresses, or cotton balls, should be provided for both the patient and the therapist to avoid damage to the conjunctiva and cornea.



Certain drugs, such as the sulfonamides, greatly increase sensitivity to ultraviolet radiation. All patients scheduled for this form of therapy should be questioned in regard to the medication they are taking so the dosage can be adjusted accordingly or the treatment deferred.

ul·tra·vi·o·let (UV, uv),

(ŭl'tră-vī'ō-let),
Denoting electromagnetic rays at higher frequency than the violet end of the visible spectrum.

Ultraviolet rays

Invisible light rays with a wavelength shorter than that of visible light but longer than that of x rays.
Mentioned in: Sunscreens

Patient discussion about Ultraviolet rays

Q. what does a sun block cream do? and what are a UV rays?

A. It blocks out harmful Ultra violet rays from the skin as the previous entries have related; however it can also block your ability to produce vitamin D. If you live in a northerly area or one that receives limited sunlight, its recommended to get at least 15 minutes of sun a day (this is probably best done with minimal sunblock) and according to personnal sun sensitivity. Another thing to keep in mind is that sunblock works best if applied 20 minutes before sun exposure.

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References in periodicals archive ?
Dr Patrick Buck, of the South Western Regional Fisheries Board, said: "Ultraviolet rays lower the immune system which means the salmon are prone to pick up disease.
"We've got to move the known understanding of the link between the sun's ultraviolet rays and skin cancer a step forward," she said.
Baron suspects that men may face an increased risk because they are more likely than women to work outdoors and thus to receive more exposure to skin-damaging ultraviolet rays. Jeopardy may intensify with age simply because years of exposure to the sun take a cumulative toll, he adds.
Experts have warned holidaymakers, especially women, to be aware of the dangers of the sun's ultraviolet rays.
However, it can also filter out harmful ultraviolet rays. Indeed, a new theoretical study suggests that ambient haze levels in even rural areas of the United States and other developed, midlatitude nations currently filter out an amount of ultraviolet light equal to or greater than the excess now leaking through the stratosphere.
There are three types of ultraviolet rays in sunlight - UVA, UVB and UVC.
A United Nations scientific panel announced last week that over the last two decades, the global ozone layer has thinned significantly during the spring and summer seasons, when people face the greatest danger from the surfs ultraviolet rays.
Despite yesterday's dip in temperature in the capital, it will be hot again over the weekend and campaigners warned that runners face a cancer risk from exposure to dangerous ultraviolet rays. They were advised to use sunblock and drink lots of water.
Ultraviolet rays permanently transmogrify these adjoining thymines into gnarled, double-looped structures that interfere with DNA replication and repair.
Another potential suspect -- increased levels of harmful ultraviolet rays penetrating the sea surface -- can, in laboratory experiments, cause zooxanthellae pigments to fade, but it does not make the coral eject the algae, Williams says.
The hole is a reduction in the ozone layer that normally shields Earth's surface from much of the sun's ultraviolet rays.

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