I am occasionally asked by my patients whether self-tanning lotions are safe. My answer is always the same; there is absolutely no question that these products are safer than sun exposure. I would much, much rather see my patients use a self-tanning lotion than damage their skin by intentionally exposing it to either sunlight or tanning beds, both of which are highly carcinogenic (cancer-causing).
Self-tanning creams contain an ingredient called dihydroxyacetone, also known as DHA (no relationship to the DHA found in fish oils). DHA is a sugar molecule that reacts with proteins in the very top layer of the skin (the stratum corneum) to bronze the skin. This bronzing effect is temporary and usually wears off in 7-10 days as the skin naturally sloughs off.
Self-tanners available in drugstores typically contain 3 to 5% DHA, a low concentration that is not thought to pose a health risk. The main risk is in assuming that the bronzing that results from their use provides much in the way of protection from the sun's harmful ultraviolet (UV) rays - it doesn't. If you use a self-tanner you still need to use a broad-spectrum sunscreen with an SPF of 30 or higher in order to protect your skin from sun damage.
Some people have questioned the safety of DHA. A study published in 2007 found an increase in free radical production for 24 hours after pig skin that had been treated with 10-20% DHA was exposed to ultraviolet light. Free radicals are molecules formed by chemical reactions that have the potential to damage cells. The effect was only seen with concentrations of DHA that are higher than those found in drugstore self-tanning products, but which may be similar to those used in "Spray tanning" booths. DHA has never been shown to cause disease in humans (apart from rare allergic reactions) and has not been classified as a cancer-causing substance (carcinogen) by any major scientific organization.
Some experts have expressed concern about the effects of DHA that is inhaled in spray tanning booths. Because DHA is intended for external, not internal use, those who choose to use spray tanning should take precautions to ensure that the DHA does not reach their eyes, mouth or lungs. That means using goggles, closing the mouth and hold one's breath while the product is being sprayed. It may also be prudent to avoid the sun or use a broad-spectrum sunscreen for 24 hours after receiving a spray tan.
For the best cosmetic results when using a self-tanner, exfoliate the skin gently before using the product, with special attention to thick-skinned sites such as the elbows and knees. Without this step, these areas will become darker than the rest of the skin. The best time to apply self-tanners is immediately after the shower, when the skin is still damp (after it has been towel-dried). Finally, wash your hands thoroughly after applying the product to prevent your palms from darkening.