Abstract
SummaryTerrestrial solar ultraviolet radiation (UVR) exerts both beneficial and adverse effects on human skin. Epidemiological studies show a lower incidence of skin cancer in people with pigmented skins compared to fair skins. This is attributed to photoprotection by epidermal melanin, as is the poorer vitamin D status of those with darker skins. We summarize a wide range of photobiological responses across different skin colours including DNA damage and immunosuppression. Some studies show the generally modest photoprotective properties of melanin, but others show little or no effect. DNA photodamage initiates non‐melanoma skin cancer and is reduced by a factor of about 3 in pigmented skin compared with white skin. This suggests that if such a modest reduction in DNA damage can result in the significantly lower skin cancer incidence in black skin, the use of sunscreen protection might be extremely beneficial for susceptible population. Many contradictory results may be explained by protocol differences, including differences in UVR spectra and exposure protocols. We recommend that skin type comparisons be done with solar‐simulated radiation and standard erythema doses or physical doses (J/m2) rather than those based solely on clinical endpoints such as minimal erythema dose (MED).
Highlights
The skin is the body’s protective barrier against environmental hazards including solar ultraviolet radiation (UVR)
It is often stated that UVA is the main cause of photoageing because it penetrates deeper into the dermis; we have shown that the action spectra for human erythema and the induction of MMP1 gene expression in human skin in vivo are similar (Tewari et al, 2012) which suggests that DNA is an important chromophore for photoageing, as has been proposed by others (Dong et al, 2008) based on in vitro and in vivo studies
Epidemiology suggests that constitutive melanin is extremely effective at preventing skin cancer in which case it might be expected to be very effective at preventing DNA photodamage and photoimmunosuppression
Summary
The skin is the body’s protective barrier against environmental hazards including solar ultraviolet radiation (UVR). Skin colour is a major human phenotypic trait and has attracted considerable research by many disciplines. The main determinants of skin colour are epidermal melanin pigments, with minor contributions from carotenoids and de/oxyhaemoglobin in dermal capillaries (Alaluf et al, 2001). An understanding of melanin is necessary to determine the role of skin colour in responses to UVR. Much of the evidence for the photoprotective role of melanin comes from the comparative epidemiology of skin cancer. The most recent review on this topic (Brenner and Hearing, 2008) focused on protection against UVR-induced DNA damage. We assess the impact of melanin and skin type on a wide range of photobiological outcomes. Our goal is to offer an illustrative rather than extensive summary of the literature and to raise awareness of possible gaps for future study
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