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Tanning is the process by which people with moderately or darkly pigmented skin produce melanin (mostly in the form of eumelanin) in their skin when they are exposed to ultraviolet radiation (UVR). The tanning reaction is complex and fully develops over the course of days and weeks, if exposure to the sun persists. Tanning involves the stimulation of melanocytes (melanin-producing cells in the skin) into a program of long-term activity. Chronic exposure to UVR can result in a near doubling of melanin content in the skin relative to baseline amounts. The degree to which tans are protective against the harmful effects of UVR has been the subject of considerable debate. Heavy tans afford little protection against UVR-induced damage to DNA relative to the amount provided by naturally dark skin. For moderately pigmented skin, however, tanning affords some protection against seasonally varying intensities of UVR because melanin production increases slowly in relation to gradually rising UVR levels in the spring and so prevents bad sunburns from being experienced during the height of summer levels of UVB. This almost certainly accounts for the evolution of tanning abilities. Naturally dark skin affords great protection against UVR because of its higher eumelanin content, the superior UVR-absorbing abilities of large clumped melanosomes, and because the eumelanin can be mobilized faster from deep in the epidermis and brought to a position closer to the surface of the skin more quickly.
Tanning is an adaptation to seasonally varying levels of UVR. It is most obvious in peoples whose ancestors lived at latitudes from 23-37 who have moderate levels of baseline (constitutive) melanin pigmentation.
The importance of the depth distribution of melanin in skin for DNA protection and other photobiological processes., , J Photochem Photobiol B, 2006 Mar 1, Volume 82, Issue 3, p.194-8, (2006)
Mechanisms of skin tanning in different racial/ethnic groups in response to ultraviolet radiation., , J Invest Dermatol, 2005 Jun, Volume 124, Issue 6, p.1326-32, (2005)