While protecting your body, skin must protect itself from the sun's ultraviolet radiation, which can cause premature aging and cancers. Sunscreens can help. Sunburn occurs when strong UV rays harm cells in the skin's outermost layer, the epidermis. Blood vessels in deeper layers dilate, turning the skin red: a sunburn. Regular exposure causes the epidermis to produce more melanin pigment in an attempt to absorb UV rays. If enough melanin accumulates, the skin darkens: a tan. Dark-skinned people have more melanin than light-skinned people and so don't burn as readily.

Active chemicals in sunscreen also filter UV rays, slowing injury and thus the sunburn and tanning reactions. Sunscreens are labeled with an SPF, or sun protection factor--a relative rating standardized by the FDA. Say that skin begins to burn after 10 minutes of exposure. When it is protected by an SPF 15 sunscreen, a comparable burn will take 15 times as long. An SPF 30 sunscreen will slow the burn for 30 times as long. But don't get cocky. Labs rate sunscreens on human subjects at a density of two milligrams per square centimeter, according to J. Frank Nash, a principal scientist at Procter & Gamble. Yet a typical beachgoer lathers up at perhaps half that concentration, halving protection. And Nash says no compound can fully stop UV penetration; there's no such thing as sunblock.

   
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