Human Skin Color - Exposure To Sun

Exposure To Sun

Melanin in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes direct and indirect DNA damage to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation.

There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin. Secondly, there is an increase in production of melanin (melanogenesis). Melanogenesis leads to delayed tanning and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin.

A person's natural skin color has an impact on their reaction to exposure to the sun. The tone of human skin can vary from a dark brown to a nearly colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans generally have lighter skin, hair, and eyes than any other group, although this is not always the case. Africans generally have darker skin, hair, and eyes, although this too is not universal. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:

Dark skin with large concentrations of melanin protects against exposure to ultraviolet light and skin cancers; light-skinned people have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore, UV-A rays from sunlight are believed to interact with folic acid in ways which may damage health.

In a number of traditional societies the sun was avoided as much as possible, especially around noon when the ultraviolet radiation in sunlight is at its most intense. Midday was a time when people stayed in the shade and had the main meal followed by a nap.

While dark skin offers better protection from intense ultraviolet light, it may result in low vitamin D levels and has led to concern that darker skinned people living at relatively high latitude, such as African Americans, may have inadequate vitamin D levels. Research shows that dark-skinned people living in Western societies have lower vitamin D levels. The explanation for low vitamin D levels in dark-skinned people is thought to be that melanin in the skin hinders vitamin D synthesis. Some recent studies have found novel evidence that low vitamin D levels among people of African ancestry may be due to other reasons, such as that black women have an increase in serum parathyroid hormone – implicated in adverse cardiovascular outcomes – at a lower vitamin D level than white women. In a large scale association study of the genetic determinants of vitamin D insufficiency in Caucasians no links to pigmentation were found.

In another recent study, the vitamin D levels of traditionally living people in East Africa with skin type 6 was actually measured for the first time; it was found that the mean calcidiol level is 115 nmol/l, which according to the researchers is much higher than was expected.

"We expected a much higher status in these people, since they do not spend most of their time behind a PC, under a roof or totally protected by their clothing habits," Luxwolda said. "However, we were surprised that their status was even much higher than the 80 nmol/l which we had expected, since this seemed most profitable from previous studies with regard to calcium homeostasis and bone fractures."