The Medical Letter on Drugs and Therapeutics
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Excessive exposure to ultraviolet (UV) radiation is associated with sunburn, photoaging, and skin cancer.1,2 Sunscreens are widely used to reduce these risks, but some questions remain about their effectiveness and safety.

UVA and UVB — UV radiation capable of injuring the skin is classified based on wavelength as UVA1 (340-400 nm), UVA2 (320-340 nm), and UVB (290-320 nm). UVA, which makes up 95% of terrestrial UV radiation, penetrates the dermis and causes long-term damage. UVB, which is mostly absorbed in the epidermis, is largely responsible for the erythema of sunburn. Both UVA and UVB can cause photoaging and skin cancer. UVB is strongest at midday and is present primarily in late spring, summer, and early autumn in temperate climates. UVA is constant throughout the day and the year and, unlike UVB, is not filtered by clear glass.

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The Medical Letter is a subscriber-funded nonprofit organization that publishes critical appraisals of new prescription drugs and comparative reviews of drugs for common diseases.

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