Air pollution‐induced tanning of human skin

Summary Background Melanism is more frequent in animals living in polluted areas on urban–industrial sites. Given that an increasing number of people are exposed to elevated air pollution levels, it is possible that environmental pollutants affect melanogenesis in human skin. Epidemiological studies have shown that exposure to traffic‐related air pollutants such as diesel exhaust particles (DEP) is associated with more clinical signs of hyperpigmentation. However, mechanistic evidence linking DEP exposure to pigmentation has been elusive. Objectives To develop an ex␣vivo skin model to allow for repetitive topical application of relevant ambient DEP, and to provide proof of concept in humans. Methods We measured skin pigmentation, melanin and pigmentation‐associated gene expression, and evaluated oxidative stress. Results Repetitive exposure of ex␣vivo skin to DEP at nontoxic concentrations increased skin pigmentation. This increase was visible to the naked eye, time dependent, and associated with an increase in melanin content and the transcription of genes involved in de novo melanin synthesis. Similarly, in healthy participants (n = 76), repetitive topical application of DEP at nontoxic concentrations increased skin pigmentation. DEP‐induced pigmentation was mediated by an oxidative stress response. After the application of DEP, epidermal antioxidants were depleted, lipid peroxidation and oxidative DNA damage were enhanced, and in a vehicle‐controlled, double‐blind clinical study DEP‐induced pigmentation was prevented by the topical application of an antioxidant mixture. Conclusions Similar to solar radiation, air pollutants cause skin tanning. As eumelanin is an antioxidant, it is proposed that this response serves to protect human skin against air pollution‐induced oxidative stress. What is already known about this topic? It has been reported that animals living in polluted environments have darker skin because they produce more melanin as a result. What does this study add? Exposure to traffic‐related air pollution causes increased pigmentation of human skin. The tanning response was seen in cultured human skin models and also in␣vivo upon exposure to nontoxic concentrations of air pollutants. What is the translational message? The results provide a mechanistic explanation to previous epidemiological studies linking chronic exposure to traffic‐related air pollution with skin hyperpigmentation. Plain language summary available online