Yes, It Can Be Too Thin

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ASSUMPTION Putting on a thin layer of sunscreen protects skin from the sun but allows a tan.

REALITY Just about all of us use sunscreen wrongly, warn researchers writing in the British Medical Journal. For sunscreen to provide the full SPF coverage promised on the bottle, it must be applied exactly as it was tested in the lab: in thick globs. The way most people do it can cut the effectiveness of sunscreen by as much as two-thirds.

ALTERNATIVE REALITY To combat the greased-chicken effect of smearing on so much sunscreen, researchers suggest covering the body with a thin layer of sunblock first, then again half an hour later. Be warned: the old rules about reapplying after swimming still hold. Plus, tans are bad for your skin, period.

--By Janice M. Horowitz