The principle tanning agent used in all self-tanning formulas is dihydroxyacetone (DHA). This ingredient does not produce a "real tan" by increasing the skin’s melanin levels, rather it creates a tanned appearance by developing a brown color on the surface of the skin.
The action of DHA is limited to the upper layer of the skin and involves a reaction between DHA and skin proteins and oxygen that produces brown compounds. This process is similar to what occurs when you slice open an apple and the surface of the apple browns when it’s exposed to the oxygen in the air.