Two different mechanisms contribute to the UV-induced darkening of the skin. Firstly the UVA-radiation generates oxidative stress which in turn oxidises pre-existing melanin. This leads to rapid darkening of already existing melanin. The second mechanism is the increased production of melanin (melanogenesis). It is a reaction of the body to photodamage from UVB. This melanogenesis is triggered by the same DNA damage that causes sunburn. Melanogenesis leads to delayed tanning. It 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.
Darkening of the skin is caused by an increased release of the pigment melanin into the skin's cells after exposure to ultraviolet radiation. Melanin is produced by cells called melanocytes and protects the body from direct and indirect DNA damage absorbing an excess of solar radiation, which can be harmful. Depending on genetics, some people can darken quickly and deeply whereas others do not darken much at all.
The ultraviolet frequencies responsible for tanning are often divided into the UVA (315 to 400nm wavelength) and UVB (280 to 315nm wavelength) ranges.
triggers the formation of CPD-DNA damage (direct DNA damage) which in turn induces an increased melanin production is more likely to cause a sunburn than UVA as a result of overexposure. The mechanism for sunburn and increased melanogenesis is identical. Both are caused by the direct DNA damage (formation of CPDs) reduced by virtually all sunscreens in accordance with their SPF is thought to cause the formation of moles and some types of skin cancer (but not melanoma) causes skin aging (but at a far slower rate than UVA.)
produces Vitamin D in human skin
causes release of preexisting melanin from the melanocytes
causes the melanin to combine with oxygen (oxidize), which creates the actual tan color in the skin seems to cause cancer less than UVB, but causes melanoma, a far more dangerous type of skin cancer than other types is blocked less than UVB by many sunscreens but is blocked to some degree by clothing is present more uniformly throughout the day, and throughout the seasons than UVB
 Health benefits
The skin produces vitamin D in response to sun exposure (in particular, UVB waves in the 285nm to 287nm range), which can be a health benefit for those with vitamin D deficiency. In 2002, Dr. William B. Grant published an article claiming that 23,800 premature deaths occur in the US annually from cancer due to insufficient UVB exposures (apparently via vitamin D deficiency). This is higher than 8,800 deaths occurred from melanoma or squamous cell carcinoma. This does not mean that sun tanning is categorically safe or beneficial. Spending several minutes in the sun is long enough to obtain your daily dose of vitamin D. Another research estimates that 50,000–63,000 individuals in the United States and 19,000 - 25,000 in the UK die prematurely from cancer annually due to insufficient vitamin D.
Another effect of vitamin D deficiency is osteomalacia, which can result in bone pain, difficulty in weight bearing and sometimes fractures. This work has been updated in Grant et al. 2005 and Grant and Garland, 2006 In addition, it was reported that in Spain, risk of non-melanoma skin cancer is balanced by reduced risk of 16 types of cancer [Grant, 2006]
According to research conducted in 2007 by Cozen, Gauderman, Islam, and Mack  , sun exposure during childhood prevents multiple sclerosis later in life....