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, and flake off. Skin cells contain melanin, and as a result of UV exposure rises to the surface and flake off as well. Therefore, a tan can be maintained only by repeated exposure to UV light.