The dynamics of pigment reactions of human skin to ultraviolet A radiation

The pigment responses of human skin to broad band UVA radiation (320-400nm) occurs in three distinct phases. The first phase includes immediate pigment darkening (IPD), the pigment that appears immediately after irradiation. The second phase involves an intermediate step, termed persistent pigment darkening (PPD), which leads to the third phase of neomelanogenesis or delayed tanning (DT). Since DT results from synthesis of new melanin, it persists beyond 5-7 days. We conducted studies on human subjects to investigate the dynamic responses of the IPD and PPD reactions to broad band UVA radiation at threshold and superthreshold doses. The threshold doses for IPD, PPD and DT were found to be approximately 1, 11 and 18 J/cm(2) respectively. The colorimetry DeltaL* value corresponding to minimal clinically perceptible pigmentation was found to be 0.8 +/-0.1. IPD appeared immediately and had an associated decay constant of approximately 1.4 minutes. At doses greater than PPD threshold, IPD reaction decayed while PPD developed indicating towards IPD being used as a substrate in the formation of PPD. This article is protected by copyright. All rights reserved