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The biological efficacy (erythema action) of the radiation is dependent on wavelength. The unit SED (standard erythema dose) is a direct measure of the erythema action of radiation. The skin by itself absorbs UV radiation depending on the thickness of stratum corneum and the degree of pigmentation. When testing SPF of a sunscreen in humans 2 mg/cm² is applied, in real life much less sunscreen is used 0.5 mg/cm². It has been suggested that the protection is reduced exponentially when a thinner layer is used. The number of sunburn cells, cutaneous DNA damage, conversion of urocanic acid, and immu-nosuppression are diminished by sunscreen use. Human studies have indicated that the number of actinic keratoses as well as squamous cell carcinomas can be reduced if sunscreens are used regularly.