The Medical Letter on Drugs and Therapeutics
Sunscreens Revisited
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Sunscreens are widely used now, but some questions remain about their effectiveness and safety.

THE SPF: IS HIGHER BETTER? — The Sun Protection Factor (SPF) is determined by the ratio of the time required under controlled conditions to develop a minimally detectable sunburn while wearing a sunscreen product to the time required without the product. It represents the fraction of erythema-producing ultraviolet light (UVL) that penetrates through the sunscreen product to reach the skin. An SPF 15 sunscreen, properly applied, transmits 1/15th (7%) of erythematogenic UV photons. An SPF 50 sunscreen transmits 1/50th or 2% and an SPF 100 transmits 1/100th or 1%. SPF 50 or 100 sunscreens are more protective than SPF 15 sunscreens, but not by much: 98% or 99% versus 93%.

EFFECTIVENESS — In the amounts customarily applied to skin, no sunscreen product provides the labeled degree of protection. The FDA ... more      

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