In one of my other roles in life, I sell skin care products to people, and I am particularly interested in the science behind many of these products and how they work. In that vein, I started looking into self-tanning products. People have been using these products as an alternative to sitting out in the sun, tanning, which we know to be highly harmful to your skin and highly correlated with the risk of developing skin cancer. Sun exposure increases skin care risks by promoting the formation of free radicals, oxidative products, and all kinds of other reactive species that react with your DNA and lead to cancer-causing changes. So wear sunscreen.
But are the self-tanner products any safer? Depends on the ingredients, largely. The most common ingredient in self-tanners is dihydroxyacetone (DHA) (https://en.wikipedia.org/wiki/Dihydroxyacetone), which reacts with the amino acids in your skin to cause a temporary darkening of color. Do you know what kind of chemical reaction this is? It is the same kind of reaction that you use when you brown meat when you are cooking, and is essentially a variation of the Maillard reaction (https://en.wikipedia.org/wiki/Maillard_reaction). Essentially then, by using self-tanners, you are ‘browning’ your own skin.
Is DHA toxic? Not generally, no, although you have to be very careful right after applying the tanner to limit your exposure to sunlight, because that reaction has made your skin much more susceptible to the bad kind of damage that the sun can cause. Occasionally people can be allergic to DHA, though, so that’s something else to be aware of.
Bottom line: If you are going to use a self-tanner, you can be confident that a DHA-containing self-tanner is a much safer way to get an aesthetically-pleasing tan than sunbathing or using UV-beds can be!