PHOTOPROTECTION DUE TO PIGMENTATION AND EPIDERMAL THICKNESS AFTER REPEATED EXPOSURE TO ULTRAVIOLET-LIGHT AND PSORALEN PLUS ULTRAVIOLET-A THERAPY

Tanning and thickening of the epidermis are cardinal defensive respons es of human skin to ultraviolet (UV) radiation that lead to increased photoprotection, Earlier studies have shown that skin pigmentation can be used to predict minimal erythema dose and minimal phototoxic dose. In this study it was calculated how much of the increase in photoprot ection after 4 weeks of repeated exposure to suberythemogenic doses of either UVA or UVB radiation sources or psoralen plus UVA (PUVA) thera py that was attributable to melanogenesis. The backs of 12 volunteers were exposed to 6 different UVA and UVB radiation sources 9 times duri ng 4 weeks. Skin pigmentation was assessed by skin reflectance measuri ng. Photoprotection was determined from the minimal erythema dose. Mel anogenesis accounted for 63-95% of the increase in photoprotection aft er 4 weeks of exposure to UVA radiation. Exposure to two UVB sources i nduced a significant increase in photoprotection but not in pigmentati on. Melanogenesis accounted only for 6-11% of the increase in photopro tection after 4 weeks of UVB exposure. The pigmentary and photoprotect ive responses to PUVA therapy were followed in 14 patients. After 2 we eks of exposure, the increase in photoprotection was significantly hig her than predicted from the increase in skin pigmentation. After 4 wee ks, melanogenesis accounted for only 36% of the increase in photoprote ction. This study shows that melanogenesis accounts for the increased photoprotection after 2 weeks of exposure to UVA radiation, but after 4 weeks other protective mechanisms occur. During suberythemal UVB exp osure and during PUVA therapy the importance of skin pigmentation in t he overall photoprotection gradually decreases during a 4-week irradia tion period.