Naughty Dog with Girls in Bikini at the Park

Girls are at the park for Sun tanning, also called bronzing, is the act of exposing the skin to ultraviolet (UV) radiation, for the purpose of darkening skin color, either during sun bathing or using artificial sources, such as tanning beds.
Excessive exposure to ultraviolet rays has detrimental health effects, including possible burn and even skin cancer.

Tanning is a natural process, whereby the skin creates the brown-colored pigment called melanin, to protect it against the harmful UV rays in sunlight, but operates equally with artificial UV radiation. There are two different mechanisms involved.
Firstly, the UVA-radiation generates oxidative stress, which in turn oxidises pre-existing melanin. This leads to rapid darkening of already existing melanin. Secondly, there is an 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.