Most sunless tanners you see on the market contain an ingredient called dihyroxyacetone (DHA for short. Not the same kind of DHA you find in omega-3 fatty acids, by the way.) Many people think that using a sunless tanner is a safer option than tanning outside. The truth is that they're not, and they may be more harmful.
First of all, it has been found that using DHA based sunless tanners can inhibit the production of vitamin D in the body. http://www.ncbi.nlm.nih.gov/pubmed/19496990 When exposed to sunlight, a response in the skin is triggered and vitamin D is created. Low levels of vitamin D have been associated with higher cancer risk, depression, and heart disease. Many people, if not most, are highly deficient in this important vitamin. Using dihyroxyacetone inhibits the production of vitamin D, thus leading to further deficiencies.
Second, products with dihydroxyacetone can have a very limited shelf life, and must be formulated in a very specific manner to be stable. The formula has to be quite acidic in order to stay stable, and that can be irritating to skin. And if not formulated properly, the DHA can break down and create strong skin irritants.
Finally, when applied to skin, it has been found to generate high amounts of free radical damage: http://www.ncbi.nlm.nih.gov/pubmed/18024196 While you may be thinking you're protecting yourself from "the sun's damaging rays" using DHA products, you're creating invisible damage to your skin that is possibly putting you at a higher risk for skin cancer.
DHA is found in even the (fake) "organic" sunless tanners. It's in the paraben-free formulations. It's everywhere. It might be mixed in with organic botanicals, and you might even see endorsements from "green experts." But DHA is DHA. It's damaging to skin, highly unstable, and inhibits one of the most important vitamins our body needs.