Dihydroxyacetone

Dihydroxyacetone, or DHA, is the active ingredient in self-tanners.

It's a simply sugar that reacts with the free amino acids in your outermost layer of skin to produce brown-colored compounds called melanoidins.

DHA does not penetrate living skin cells, does not interact with melanocytes, and does not affect actualy melanin production.

There's a "safety controversy" that largely stems from misinterpreted studies:

Once concern is that DHA can generate unstable molecules that can damage cells (free radicals) when exposed to sunlight. This only happens in the outermost layer of dead skin cells and wearing SPF on top takes care of it.

The DNA damage claim comes from lab studies that doused living skin cells in much higher concentrations of DHA than you'd ever find in a self-tanner. That's not really a meaningful comparison to putting self-tanning lotion on your skin.

Regulatory bodies around the world, including the EU's Scientific Committee on Consumer Safety (SCCS) and the U.S. FDA consider it safe for use in cosmetics when applied topically (maximum 10%, and most self-tanners contain between 3-5%).