What is the tanning process?

UVB (from the sun or in a low-pressure indoor tanning unit) stimulates the melanocytes in the upper skin layer (called the epidermis) which then produce melanin. The pale pink melanin granules formed in the melanocytes are stored around the core of the keratin cells. In this manner the pigment granula protects the sensitive DNA located inside the nuclei without impeding the other parts of the cell receiving ultraviolet light. UVA then darkens the melanin, thereby giving the skin a dark appearance. A tan gradually fades as the skin cells migrate to the surface.

Why does a tan disappear?

The "tan", or pigmentation process, occurs in the epidermis, the top skin layer. The epidermis replaces all its skin cells every 28-30 days. Cells in the inner portion of the top skin layer divide themselves, migrate to the surface, gradually die and slough off. Skin cells contain melanin, and as a result of UV exposure, rise to the surface and flake off, Therefore, a tan can only be maintained by repeated exposure to UV light.