How does tanning occur?

In general, tanning is the process whereby melanocytes produce melanin, which gives the skin a red or brown pigment.

There are two mechanisms by which this happens:

1) UV-A radiation creates oxidative stress, which oxidizes existing melanin, leading to darkening of the skin. UV-A can also cause the redistribution of melanin via release from melanocytes. This production of melanin helps to prevent sunburn due to UV-B exposure.

2) UV-B radiation triggers an increase in the production of melanin (melanogenesis), which is the body's reaction to photodamage (bonding of pyrimidine to form dimers...a type of radiation-induced chemical damage to the bases of DNA and RNA). Melanogenesis leads to delayed tanning, becoming visible about 72 hours later.

While UV-A tanning is much more short-lived and serves as a protection mechanism against UV-B, at least some DNA photodamage must be produced to yield the long-term effects of tanning created by UV-B-induced melanogenesis.