Sun 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 artificial sources, such as a tanning lamp found in indoor tanning beds.

Moderate exposure to direct sunlight contributes to the body’s production of melanin and vitamin D. Still, excessive exposure to ultraviolet rays has adverse health effects, including sunburn, increased risk of skin cancer, depressed immune system function and accelerated aging of the skin. Some people tan or sunburn more quickly than others. This may result from different skin types and natural skin color, and these may be a result of genetics. The term “tanning” has a cultural origin, arising from the color tan. Its origin lay in the Western culture of Europe when it became fashionable for young women to seek a less pale complexion.

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 and other direct and indirect DNA damage to the skin. 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 melanin, the skin color darkens. The tanning process can be triggered by natural sunlight or artificial UV radiation, which can be delivered in frequencies of UVA, UVB, or both.

There are two different mechanisms involved in producing 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. Therefore, skin darkening from UVA exposure does not lead to significantly increased melanin production or protection against sunburn.

In the second process, triggered primarily by UVB, there is an increase in melanin (melanogenesis) production, which is the body’s reaction to direct DNA 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 created by increased melanogenesis typically lasts for a few weeks or months, much longer than the tan 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.

However, to cause true melanogenesis-tanning through UV exposure, some direct DNA photodamage must first be produced, which requires UVB exposure.