Here is a little background info on sunless tanning (dha) i cant link the site so pasted and copied. x
Uses[edit]
DHA was first recognized as a skin coloring agent by German scientists in the 1920s. Through its use in the X-ray process, it was noted as causing the skin surface to turn brown when spilled.
In the 1950s, Eva Wittgenstein at the University of Cincinnati did further research with dihydroxyacetone.[7][8][9][10] Her studies involved using DHA as an oral drug for assisting children with glycogen storage disease. The children received large doses of DHA by mouth, and sometimes spat or spilled the substance onto their skin. Healthcare workers noticed that the skin turned brown after a few hours of DHA exposure.
Eva Wittgenstein continued to experiment with DHA, painting liquid solutions of it onto her own skin. She was able to consistently reproduce the pigmentation effect, and noted that DHA did not appear to penetrate beyond the stratum corneum, or dead skin surface layer (the FDA eventually concluded this isn't entirely true[11]). Research then continued on DHA's skin coloring effect in relation to treatment for patients suffering from vitiligo.
This skin browning effect is non-toxic[citation needed], and similar to the Maillard reaction. DHA reacts chemically with the amino acids in the skin, which are part of the protein containing keratin layer on the skin surface. Different amino acids react to DHA in different ways, producing different tones of coloration from yellow to brown. The resulting pigments are called melanoidins. These are similar in coloration to melanin, the natural substance in the deeper skin layers which brown or "tan", from exposure to UV rays.