Sunburn protection by longwave ultraviolet radiation-induced pigmentation.

The role of melanin pigment in sunburn protection was investigated. Deep tans were induced over the backs of volunteers with repeated exposure to longwave ultraviolet radiation (UV-A). Melanogenesis was stimulated without an appreciable thickening of the stratum corneum. Two to three times the minimal erythema dose was required to produce redness in UV-A-tanned skin. Transmission studies through isolated corneum sheets revealed that specimens from tanned skin were about twice as efficient in filtering sunburn rays as stratum corneum from untanned skin. Tanning with UV-A does not provide a substantial resistance against sunburn.