A. The ultraviolet portion of sunlight causes some skin cells to make more of a dark pigment called melanin, says Nihal Ahmad, an associate professor of dermatology at the UW School of Medicine and Public Health. Melanin has a protective role, because after formation it blocks some ultraviolet light.

The first exposure to ultraviolet also causes free radicals damaging parts of atoms or molecules to form in the skin. Free radicals can oxidize and darken melanin, which is a second mechanism that causes tanning.

People whose skin is dark due to a high level of melanin are protected against some ultraviolet radiation, but with too much sun and/or too little melanin, sun exposure can cause the redness and swelling called sunburn.

When free radicals damage the DNA in skin cells, those cells may either repair their DNA or commit suicide through the process of programmed cell death. Otherwise, they may form colonies of abnormally reproducing cells. Those cells have started on the road toward cancer, especially if they receive further doses of ultraviolet radiation.

To prevent cancer and premature aging of the skin, people, especially those with fair skin, are advised to avoid intense sun, and use protective clothing and strong sunscreen.

-- Produced in cooperation with University Communications.

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