Melanin differentially protects from the initiation and progression of threshold UV-induced erythema depending on UV waveband

Background/purpose: This study aimed to determine the relationship between various measures of constitutive skin pigmentation and erythema caused by solar-simulated UV (ssUV), 290 and 310 mn UV. Methods: Skin pigmentation was assessed clinically by skin typing as well as objectively by measurement of the melanin index (MI) by reflectance spectroscopy. Subjects having Fitzpatrick skin types I-IV were exposed to graded doses of ssUV and either narrow-band 310 mn (n = 70) or 290 nm (n = 69) UV, and assessed 24 h after exposure. Minimal erythema dose (MED) was assessed visually as the lowest dose that caused minimally perceptible erythema. Susceptibility to further development of erythema with higher exposure doses was measured by the gradient of erythema dose-response curves. This was determined by linear regression using reflectance spectrometry data beyond the MED. Results: Although there was considerable variation within each skin type, MI and ssUV MED increased with increasing Fitzpatrick skin type. MI correlated with ssUV MED and 310 nm UV MED, but not 290 urn UV MED. There was also a significant negative correlation between MI and erythema dose-response gradients caused by ssUV, 310 and 290 nm UV. Conclusion: Melanin situated near the basal epidermis may not protect from the initial development of threshold erythema caused by 290 mn UV because it penetrates poorly past the stratum corneum and is not well absorbed by melanin in vivo compared with 310nm UV. Higher erythemal 290mn UV doses may reach basal epidermal melanin, which may then afford protection against further 290 mn UV erythema.