Tanning of the Skin

Tanning is the natural reaction of human skin to exposure to UV light. There are two parts to this process:

  • UVA activates existing melanin in the skin, darkening it and producing a slight darkening of the skin. This effect appears quickly, typically within an hour or two, but disappears almost as quickly once exposure ceases.
  • UVB triggers the melanocytes in the epidermis to produce more melanin, producing a much more pronounced darkening of the skin than that produced by UVA exposure. This type of tan takes a couple of days to appear, but lasts for several weeks, until the additional melanin is lost through the natural cycle of epidermis replacement.

The melanin that causes the darkening of the natural skin colour helps to prevent penetration of the skin by further UV radiation, protecting it from further damage. The protection is, however, only limited, typically equivalent to a SPF 3 sunscreen.

The UV blocking effect of a tan also reduces the production of vitamin D in the skin Thus although developing a tan is in no way necessary to ensure adequate vitamin D production, having a tan then means that more sun exposure is needed to maintain adequate vitamin D levels.

This effect also helps to explain the geographical variation in natural skin colour among the various races of the human species: it is nature’s balancing act between protection against UV damage (darker skin colour) and adequate vitamin D production (lighter skin colour).

Thus a darker skin colouration developed over the centuries among those races that migrated to areas near the equator, where sun intensity is high and UV protection is more important, while a lighter skin colouration developed in those that migrated away from the equator, where less UV protection is required but it is more difficult to generate adequate vitamin D levels.

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