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Sun tanning or simply tanning is the process whereby skin color is darkened or tanned. It is most often a result of exposure to ultraviolet (UV) radiation from sunlight or from artificial sources, such as a tanning bed. People who deliberately tan their skin by exposure to the sun engage in a passive recreational activity of sun bathing. Some people use chemical products which can produce a tanning effect without exposure to ultraviolet radiation. Moderate exposure to the sun contributes to the production of vitamin D by the body, but excessive exposure to ultraviolet rays has negative health effects, including sunburn and increased risk of skin cancer,〔(【引用サイトリンク】title=Ultraviolet (UV) Radiation )〕 as well as depressed immune system function and accelerated aging of the skin. Some people tan or sunburn more easily than others. This may be the result of different skin types and natural skin color, and these may be a result of genetics. Several cases of tanning addiction have been reported. Though the mechanism by which tanning addiction occurs is unknown, some evidence indicates that the release of endorphins during the tanning process causes the pleasurable effects that underlie the addiction. The term "tanning" has a cultural origin, arising from the color tan. Its origin lies in the Western culture of Europe when it became fashionable for young women to seek a less pale complexion (see Cultural history below). ==Tanning process== Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanocytes produce two types of melanin: pheomelanin (red) and eumelanin (very dark brown). Melanin protects the body by absorbing solar radiation. 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 the UVB in sunlight can also cause sunburn. The tanning process can also be created by artificial UV radiation, which can be delivered in frequencies of UVA, UVB, or a combination of both. There are two different mechanisms involved in production of a tan by UV exposure: Firstly, UVA radiation creates oxidative stress, which in turn oxidises existing melanin and leads to rapid darkening of the melanin. UVA may also cause melanin to be redistributed (released from melanocytes where it is already stored), but its total quantity is unchanged. It was found empirically that skin darkening from UVA exposure does not lead to greatly increased production of melanin or protection against sunburn. In the second process, triggered primarily by UVB, there is an increase in production of melanin (melanogenesis), which is the body's reaction to direct photodamage (formation of pyrimidine dimers) from UV radiation.〔 Melanogenesis leads to delayed tanning, and typically becomes visible two or three days after exposure.〔 The tan that is created by increased melanogenesis typically lasts for a few weeks or months, much longer than the tan that is caused by oxidation of existing melanin, and is also actually protective against UV skin damage and sunburn, rather than simply cosmetic. However, in order to cause true melanogenesis-tanning by means of UV exposure, some direct DNA photodamage must first be produced, and this requires UVB exposure (as present in natural sunlight, or sunlamps that produce UVB). The ultraviolet frequencies responsible for tanning are often divided into the UVA and UVB ranges. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sun tanning」の詳細全文を読む スポンサード リンク
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