inflammation of the skin caused by actinic rays from the sun or artificial sources. Moderate exposure to ultraviolet radiation is followed by a red blush, but severe exposure may result in blisters, pain, and constitutional symptoms. As ultraviolet rays penetrate the skin, they break down collagen and elastin, the two main structural components of the skin, a process that results in the wrinkled appearance of sun-damaged skin. In addition, the sun damages the DNA of the exposed skin cells. In response, the cells release enzymes that excise the damaged parts of the DNA and encourage the production of replacement DNA (a process that can go wrong and result in skin cancer). At the same time, the production of melanin increases, darkening the skin. Melanin, the pigment that gives skin its color, acts as a barrier to further damage by absorbing ultraviolet light. A suntan results from this attempt by the skin to protect itself. Light-skinned persons and infants are especially susceptible to ultraviolet rays because they lack sufficient protective skin pigment. Certain diseases and drugs may also increase photosensitivity.

Due to the increase in the incidence of skin cancer and the effects of ozone layer depletion, more attention is being placed on protecting the skin from the sun's ultraviolet rays with broad spectrum sunscreens or clothing. Broad spectrum sunscreens block both UVA and UVB rays (two of the three bands of ultraviolet radiation). The relative UVB protection of a sunscreen is indicated by its SPF (sun protection factor) number; a higher number indicates a more effective sunscreen. Some products may contain opaque formulations of zinc oxide or titanium dioxide that physically block all rays.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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