Real tans come from UV-triggered pigment production in living skin; that same UV exposure increases the risk of skin cancer.

Bronzers are simply colored coatings that sit atop the skin and can be washed away.

How, then, to create a fake tan that stays put, without exposing people to UV light and skin cancer risk?

The trick is not to get brown pigment into the skin directly. Brown pigments are usually large molecules, which do not penetrate the skin barrier easily.

Instead, fake-tan products smuggle in something much smaller, then generate the pigment chemically once that smaller molecule is already inside the outer skin layer.

How to smuggle in a molecule

The outermost layer of skin, the stratum corneum, consists of dead skin cells embedded in lipids (fats). While it is an excellent barrier, some small molecules can still get through.

The active ingredient in most fake-tan products is dihydroxyacetone, or DHA—a tiny sugar molecule found naturally in our bodies.…