The skin is made up of two main layers: the epidermis on the outside and the dermis on the inside. Whether you are talking about sun tanning or self-tanning, the epidermis is where the action occurs. The epidermis is also made up of layers. The deepest layer of the epidermis, called the stratum basale (basal layer), is affected during sun tanning. The stratum corneum (horny layer) is the outermost layer of the epidermis -- it is this layer that is affected by most sunless-tanning products.
 

There are several different kinds of sunless-tanning products available today. People have been able to pour on a tan since 1960, when Coppertone® came out with the first sunless-tanning product -- QT® or Quick Tanning Lotion. If you are old enough to remember this, then you are probably thinking of the incredibly orange hue this lotion produced. Since then, there have been several advancements made on the sunless-tanning front. These days, you can find tanning pills, sunless- or self-tanners and bronzers. You can smooth, swipe or spray on a light bronze glow or a deep, dark tan. Many of these products take 45 minutes to one hour to start taking effect, and once you factor in drying time, you could be looking at about three hours spent achieving that sun-free tan.

According to the American Academy of Dermatology, the most effective products available are sunless- or self-tanning lotions that contain dihydroxyacetone (DHA) as the active ingredient. DHA is a colorless sugar that interacts with the dead cells located in the stratum corneum of the epidermis. As the sugar interacts with the dead skin cells, a color change occurs. This change usually lasts about five to seven days from the initial application.

Every day, millions of dead skin cells are sloughed off or worn away from the surface of your skin. In fact, every 35 to 45 days, you have an entirely new epidermis. This is why tans from sunless- or self-tanning lotions will gradually fade -- as the dead cells are worn away, so is your tan. For this reason, most of these products suggest that you reapply the sunless- or self-tanner about every three days to maintain your "tan."

Although gels, lotions or sprays that contain DHA are said to be the most reliable and useful, there are dozens of other types of products on the market. Tanning accelerators -- lotions or pills that usually contain the amino acid tyrosine -- claim that they stimulate and increase melanin formation, thereby accelerating the tanning process. At this time, there is no scientific data available to support these claims.

Another sunless-tanning product is a tanning pill that contains canthaxanthin, which is most commonly used as a color additive in certain foods. Although the FDA has approved the use of canthaxanthin in food, it does not approve its use as a tanning agent. When used as a color additive, only very small amounts of canthaxanthin are necessary. As a tanning agent, however, much larger quantities are used. After canthaxanthin is consumed, it is deposited all over your body, including in your skin, which turns an orange-brown color. These types of tanning pills have been linked to various side effects, including hepatitis and canthaxanthin retinopathy, a condition in which yellow deposits form in the retina of the eye.

Another popular form of sunless tanning is the bronzer. These powders and moisturizers, once applied, create a tan that can easily be removed with soap and water. More like make-up, these products tint or stain your skin only until they are washed off.

It's important to remember that most of these products, unless they contain an added sunscreen, will not protect you from the sun's UVA and UVB rays. Even products that do contain a sunscreen won't be of much help, since they lose their efficacy within hours of application. So, if you're planning to head outside to show off your new glow, be sure to apply some extra sunscreen.