Exposure to the sun’s ultraviolet radiation darkens granules of the pigment melanin in the surface layers of the skin.
Part of the radiation also stimulates pigment-producing cells, called melanocytes, in deeper layers of the skin, causing a delayed-reaction tan, about three days after exposure.
When the top layers of skin wear off, so does the tan.
All adults have about 60,000 melanocytes per square inch of skin. Skin color is determined by how much pigment the cells make and what color it is; it ranges from black to light tan.
Other factors that stimulate melanin production include X rays, heat, certain chemicals, hormones, and drugs (usually in combination with sun exposure), and inflammatory skin diseases.
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 leathery appearance.
Some newer sunblocks protect against both kinds of radiation, but nothing works as well as avoiding midday sun exposure altogether and wearing a hat and long sleeves.