As a tanner, you've probably always been curious about exactly how your skin tans. The process is really quite simple, and works the same whether you tan indoors or outdoors.

Ultraviolet light is the catalyst, and a pigment in your skin called melanin does the rest.

Tanning takes place in the skin's outermost layer, the epidermis. About five percent of the cells in your epidermis are special cells called melanocytes. When exposed to ultraviolet B light (short wave ultraviolet), melanocytes produce melanin-the pigment which is ultimately responsible for your tan. The pinkish melanin travels up through the epidermis and is absorbed by other skin cells. When exposed to ultraviolet A light (longer wave), the melanin oxidizes or darkens. This darkening is your skin's way of protecting itself against too much UV light.

Everyone has the same number of melanocytes in their bodies-about five million. But your heredity dictates how much melanin your body's melanocytes naturally will produce. For example, the skin of African Americans contain enough melanin to create a black or brown skin color, while the skin of Caucasians has less melanin and is pale.

In order to most effectively avoid overexposure, a tan should be acquired gradually, according to the guidelines prescribed by your salon professional. A sunburn, or erythema, occurs when too much ultraviolet light reaches the skin and disrupts the tiny blood vessels near the skin's surface.

Why does a tan fade? Cells in the epidermis' germinative layer (also called the living epidermis) are constantly reproducing and pushing older cells upward toward the horny layer (dead epidermis), where they are sloughed off in about one month. As your skin replaces its cells, the cells laden with melanin are removed. So the tanning process must continue with the new cells.

The Epidermis - Your skin's epidermis consists of two layers: the germinative layer (sometimes called the "living" epidermis) and the horny layer (the "dead" epidermis). When exposed to ultraviolet light, melanocytes in the germinative layer produce melanin which is absorbed by surrounding cells. This creates a protective barrier from ultraviolet light reaching deeper, more sensitive layers of the skin. This whole tanning process is the body's own natural defense against sunburn and skin damage.

Short and Long Term Tanning

The skin's protective responses to ultraviolet radiation include a thickening of the stratum corneum, a thickening of the epidermis and a generation of photoproducts such as pigmentation. The skin generates photoproducts either in short-term events that occur within minutes, or in long-term events that occur over days or weeks.

The tanning response to ultraviolet radiation is unique in that its protective effects are generated in both short and long term events. The long term tanning event, delayed tanning, is commonly known as melanogenesis. The short term tanning, called immediate tanning or immediate skin pigment darkening, is less known and less understood.

Normal human skin can be classified into six skin types, determined by pigmentation response. Skin Type I cannot achieve a tan, even with moderate and repeated exposures. Instead, this skin type creates "pheomelanin," a pigment that has little color and is ineffective in protecting the skin against ultraviolet damage. Skin Types II and III can tan, but will generate only a light brown color. These skin types create "eumelanin," the pigment we know to productive against damage from subsequent ultraviolet exposures. Skin Types IV and V tan easily and profusely with minimal exposure. Skin Type VI is extremely pigmented, even in the absence of ultraviolet exposure.

Long Term Tanning

Delayed tanning includes an increase in the number of melanosomes, an increase in the activity of tyrosinase (the enzyme key to melanin production) and an increase in the synthesis of new melanin. Because the process is elaborate, delayed tanning has gradual onset. Some have reported delayed tanning to occur in 48 hours with exposure to extreme amounts of ultraviolet radiation; modest exposure results in more gradual tanning. UVB radiation induces more delayed tanning than does UVA.

Short Term Tanning

Scientists believe immediate tanning is the result of changes in existing melanosomes or melanin and that it is a passive chemical process instead of an active biological process. The major reason immediate tanning is considered a passive process is that it can be induced in dead skin samples. Even skin that has been frozen or preserved with a fixative can demonstrate immediate tanning.