Exposure to the sun's ultraviolet radiation darkens granules of the pigment melanin in the surface layers of the skin. When the top layers of skin wear off, so does the tan. All adults have about 60,000 melanocytes per square inch of skin.
Dermatologists warn that tanning goes hand in hand with skin aging and a higher risk of skin cancer.
Ultraviolet B, or UVB, radiation, the so-called burning rays, is the kind most sun products protect against. They usually offer little protection against the ultraviolet A, or UVA rays, the so-called tanning rays. These are the rays that affect deeper layers of the skin, causing the breakdown of supporting tissue, resulting in premature aging, wrinkling and a leatherlike appearance. Some newer sun blocks protect against both kinds of radiation, but nothing works as well as avoiding midday sun exposure altogether and wearing a hat and long sleeves.