The impact of skin colour on human photobiological responses

Abstract

Terrestrial 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).

Keywords: melanin; photobiology; skin colour; skin type; ultraviolet radiation.

Figure 1

The biosynthesis of the two main melanin pigments in human melanocytes. Downstream of a functional MC1R receptor via αMSH/ACTH stimulation, the black–brown eumelanin is produced, whereas pheomelanin (yellow/red) is derived from the antagonistic action of agouti signalling protein (ASIP) on MC1R in the presence of cysteine (Ito and Wakamatsu, 2008).

Figure 2

Incidence of Melanoma by race/ethnicity in United States of America, 2007–2011. Incidence rates are from age‐adjusted data (SEER Cancer Statistics Factsheets).