Protection against UV-B by UV-A-induced tan.

Suntan is known to be protective against sunburn resulting from medium-wavelength ultraviolet radiation. Protection against UV-B by UV-A-induced pigmentation was studied by the evaluation of the erythema produced and by histologic alterations in the epidermis, including sunburn cell counts. Six subjects were deeply tanned with UV-A (mean cumulative dose, 700 joules/sq cm). The minimal erythemal dose to UV-B (300 +/- 5 nm) was established for each subject in normal skin and in UV-A-tanned areas. The UV-A tan provided measurable protection against UV-B-induced erythema. Following UV-B irradiation, 36 to 93 sunburn cells were counted per 10 mm of epidermal surface length in histologic sections of untanned skin as compared with four to seven sunburn cells in UV-A-tanned skin.