A tan is a a sign of injury. Tanned skin will forever contain cells whose genetic structures have been permanently damaged by the sun. There is no such thing as a healthy tan.
The sun gives off invisible rays of ultraviolet light. Ultraviolet B (UVB) rays are short, high-energy wavelengths that are absorbed by the epidermis, the outermost layer of the skin. When you burn, the skin responds to UVB rays by producing chemicals called inflammatory mediators, some of which seep down into the dermis, the skin's middle layer. These chemicals irritate the tiny blood vessels in the dermis, which swell and create the surface redness of the burn.At the same time, the UVB rays affect the genetic material of the epidermis, which causes the damage that may lead to skin cancer. Other UVB rays can affect the immune system and interfere with the skin's ability to repair itself. Finally, UVB radiation attacks the skin's melanocytes (pigment cells). The melanocytes react by stepping up production of melanin and sending melanasomes to the skin's surface to act as a filter against the sun's rays which are actually damaging the DNA of the pigment cells. This kind of genetic damage causes both freckling and the mottled brown of age spots. More seriously, it contributes to the development of melanoma and other skin cancers.
Ultraviolet A (UVA) rays - longer than UVB rays - can also do lasting damage. They penetrate the skin more deeply than UVB rays, affecting the DNA of the cells in the dermis, attacking cell membranes, and changing the proteins that make up collagen and elastin, which support the skin's fibrous structure. By undermining these parts of the skin, UVA rays lead directly to wrinkles and sagging of the skin. They also contribute to the loss of support for the skin's tiny blood vessels, which become permanently dilated. This shows up as a general ruddiness or visible spider veins on the nose, cheeks and chin. UVA rays also play a role in the development of skin cancer.
-American Skin Association