Question

Sun tanning is a chemical process. Why?

Answer 1

Sun tanning is a process where the skin color is darkened or tanned. It occurs as a result of exposure to ultraviolet (UV) rays from sunlight or from artificial sources like tanning lamps found in indoor tanning beds. Excessive exposure of the body to ultraviolet rays causes sunburn and increases the risk of skin cancer.

Complete answer:
Sun tanning is a chemical process because vitamin D and melanin are produced when the body is exposed to sunlight.
Melanin is a natural substance produced by the body that gives brown color to skin, hair, and the iris of the eyes. Melanin absorbs harmful UV rays and protects the skin from UV damage. Excessive exposure of the body to sunlight induces excessive melanin production (melanogenesis) as a defense mechanism.
The epidermis is the site for vitamin D synthesis. During exposure to sunlight, $7$-dehydrocholesterol present in the epidermis undergoes UVB photolysis and isomerization to form vitamin D$3$ or cholecalciferol. This vitamin D$3$ is then transported to the kidney where it gets converted into the biologically active form of vitamin D or calcitriol.

Note:
Sun tanning occurs in two phases. The first phase is immediate pigment darkening (IPD). In this phase, the skin shows rapid darkening during UVA exposure and hence has a maximum visible effect. The second phase of delayed tanning is increased melanin production which becomes visible $3$ days after UV exposure. Delayed tanning needs a considerable amount of exposure and intensity to both UVA and UVB rays from the sun.