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How does sunscreen work? -JAMIE REED, OKLAHOMA CITY, OKLA.

That depends on which kind you're using. Basic sunscreen contains chemicals that absorb the energy in ultraviolet rays, then emit it back as lower-frequency heat waves. The higher its SPF, the more UV rays it can absorb and re-emit--but even an SPF of 30 will let a few rays by. (SPFs higher than 30 aren't substantially more effective, so save your money.) For total protection, there's zinc oxide or titanium oxide, the opaque white goo lifeguards smear on their noses. It physically blocks the sunlight, rather than absorbing it. But unless you're going for that full-body-paint look, you're better off with the SPF 30 and a wide-brim hat.

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