Randall R. Wickett, professor of pharmaceutics and cosmetic science at the University of Cincinnati College of Pharmacy, explains.

Although many people still desire a suntan, we now understand just how damaging exposure to the sun can be. Repeated sun exposure greatly increases skin wrinkling and may even lead to skin cancers. Adding wrinkles while trying to improve appearance is obviously counterproductive and has led to the popularity of so-called sunless tanning products.

The active ingredient in sunless tanners is the compound dihydroxyacetone (DHA). DHA is approved by the Food and Drug Administration for use in externally applied cosmetics intended to impart color to the skin. A three-carbon sugar, DHA reacts with amino groups in the proteins of the top layer of the skin, called the stratum corneum (SC). The first step is known as the Maillard reaction, the same process that leads to the caramelization of sugars and the browning of bread. But the complete details of the subsequent reactions are still not fully worked out.

Sunless tans are not exactly the same color as natural tans and do not absorb ultraviolet light in the range that causes sunburn. They therefore do not protect against sunburn. The products of sunless tanning also absorb a little less red light than natural skin pigments do and may lead to orange coloration in some cases. This was particularly true of earlier formulations, hence the orange appearance of "quick tans" in the past. Modern products contain lower DHA concentrations, leading to a "tan" that is more natural looking, albeit not as dark. Some products also contain the natural sugar erythrulose, which adds slightly more red for a more natural color.

Sunless tanners are not without drawbacks. Failure to apply the product evenly can lead to inconsistent color. And some users experience an odor that develops during the tanning process. Sometimes described as "starchy," it is a sign that the product is working. But because the odor actually changes during the development of the tan, it is difficult to mask with fragrances.

Sunless tans are not permanent because only the SC is stained and we continually shed this layer of the skin through the process of exfoliation. Typically it takes about three weeks for the SC to "turn over" completely. Use of exfoliating scrubs or exfoliating products such as alpha- or beta-hydroxy acids can speed the exfoliation process and thereby shorten the duration of a sunless tan.

Sunless tans provide an alternative to sun exposure and tanning booths. They are likely to increase in popularity as we learn even more about the damaging effects of ultraviolet light on skin.

Answer originally posted on April 12, 2004.