07/22/2023**Tanning process

Tanning process

Melanin is a natural pigment 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 ultraviolet radiation. Excessive UV radiation causes sunburn along with other 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. The tanning process can be triggered by natural sunlight or by artificial UV radiation, which can be delivered in frequencies of UVA, UVB, or a combination of both.[5] The intensity is commonly measured by the UV Index.[6]

There are two different mechanisms involved in the production of a tan by UV exposure: Firstly, UVA radiation creates oxidative stress, which in turn oxidizes 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. Skin darkening from UVA exposure does not lead to significantly increased production of melanin or protection against sunburn.[7] In the second process, triggered primarily by UVB, there is an increase in production of melanin (melanogenesis),[8] which is the body's reaction to direct DNA photodamage (formation of pyrimidine dimers) from UV radiation.[9] Melanogenesis leads to delayed tanning, and typically becomes visible two or three days after exposure.[8] 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. Typically, it can provide a modest Sun Protection Factor (SPF) of 3, meaning that tanned skin would tolerate up to 3 times the UV exposure as pale skin.[10] 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).[citation needed] The ultraviolet frequencies responsible for tanning are often divided into the UVA and UVB ranges.

UVA[edit]

Ultraviolet A (UVA) radiation is in the wavelength range 320 to 400 nm.[11] It is present more uniformly throughout the day, and throughout the year, than UVB. Most UVA is not blocked by the atmosphere's ozone layer. UVA causes the release of existing melanin from the melanocytes to combine with oxygen (oxidize) to create the actual tan color in the skin.[citation needed] UVA is blocked less than UVB by many sunscreens, but is blocked to some degree by clothing. UVA is known both to cause DNA damage and to be carcinogenic. However, it operates not by inducing direct DNA damage, but by producing reactive oxygen species which damage DNA indirectly. UVA (see above) induces a cosmetic tan but little extra melanin protection against sun damage, sun burn, or cancer.[citation needed]

UVB[edit]

Ultraviolet B (UVB) radiation is in the wavelength range 280 to 320 nm. Much of this band is blocked by the Earth's ozone layer, but some penetrates. UVB:

• triggers the formation of CPD-DNA damage (direct DNA damage), which in turn induces an increased melanin production.[9]

• is more likely to cause a sunburn than UVA as a result of overexposure. The mechanism for sunburn and increased melanogenesis is identical.[12] Both are caused by the direct DNA damage (formation of CPDs).

• produces Vitamin D in human skin.

• is reduced by virtually all sunscreens in accordance with their SPF.

• is thought, but not proven, to cause the formation of moles and some types of skin cancer.

• causes skin aging (but at a slower rate than UVA).[citation needed]

• stimulates the production of new melanin, which leads to an increase in the dark-colored pigment within a few days.[13]