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Dabbing on a bit of sunscreen as soon as you head for the outdoors will neither adequately protect your skin from the sun's aging effects nor reduce your risk of developing skin cancer, according to a dermatologist at New York University Medical Center.

"Sunscreens as they are currently formulated are quite effective, but they work only when they are applied correctly," said Dr. Darrell Rigel, clinical assistant professor of dermatology.

An article in an upcoming issue of the center's Health Letter lists some requirements for optimum sunscreen use.

"First, use a product with an SPF (sun protection factor) of 15 or higher," Rigel said. "SPF indicates an equivalence between the number of hours of sun exposure with the product and one hour of exposure with unprotected skin. SPF 15 means that 15 hours of sun exposure with that sunscreen is equal to one hour with no protection.

In theory, the higher the SPF, the more protection, but there is less difference in protection higher up the scale. "SPF 30 provides only about 3 percent more protection than SPF 15. Using a product with SPF 15 or higher is recommended, but there is no significant gain above 15."

People must be sure to apply enough sunscreen, Rigel said. "It takes about one ounce to cover the average body adequately. That includes such vulnerable and often ignored areas as the backs of hands and tops of feet, bald spots, the back of the neck, the ears and the nose."

Rigel further advised that sunscreens be put on 20 minutes before going out in the sun. "It takes a while for the active ingredients to be taken into the skin," he said. "Unlike sunblocks _ zinc oxide, for example _ which sit on the skin and provide a physical barrier, sunscreens work beneath the skin to screen out ultraviolet rays. Sunbathers who do not apply sunscreen until they are on the beach leave skin unprotected for about 20 minutes, in which time much sun damage can be sustained."

Sunscreens should not just be used at the beach. "Children spend a lot of time outdoors during spring and summer, often in abbreviated clothing. It is becoming clear that the sunburns of the childhood and teen years are the ones that increase the risk of skin cancer later in life."