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
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