Dihydroxyacetone
In plain terms
The active ingredient in nearly every sunless tanner and spray tan. It reacts with the outermost dead-skin proteins to turn them brown, a surface color change that fades as those cells shed. It gives no protection from the sun.
What the evidence shows
Dihydroxyacetone, or DHA, colors skin by reacting with the dead outer layer, and how safe it is depends almost entirely on whether you rub it on or breathe it in. As a lotion, mousse, or cream, the reaction stays in the surface cells that are about to flake off, little of it penetrates, and the FDA allows it for this external use. The line the FDA draws is exactly where the concern sits: it has approved DHA only for use on external skin, keeping it clear of the eyes, lips, other mucous membranes, and the airways, and it has never cleared it for the spray-booth and airbrush uses that are hard to keep out of the nose and mouth. That caution has support in the lab. In cell studies DHA damages DNA at high concentrations, and a 2025 mouse study found that inhaling it caused lung inflammation and, with repeat exposure, scarring of the lung. None of that bears on a cream you smooth over your arms, but it does argue against inhaling the aerosol in a tanning booth without protecting your airways. One more point holds for every form: a sunless tan is only a stain, it adds no sun protection, and DHA-treated skin briefly forms more free radicals under UV, so sunscreen still matters. So the split is by delivery: a rub-on self-tanner is a low concern, while breathing the spray is the use to avoid or protect against.
On the label
Listed as dihydroxyacetone or DHA, the coloring agent in self-tanners and spray tans. A rub-on lotion or foam is the low-concern way to use it; in a spray booth, protect your eyes, nose, and mouth, and remember it is not sunscreen.