Spray tanning is a method of self tanning (or sunless tanning) in which you spray a fine mist on your body. This mist has an ingredient called Dihydroxyacetone (DHA) in it that interacts with the chemistry of your own skin to make it tan, or bronze. If bread is toasted and potatoes are roasted, the chemical reaction is similar. The amino acids on the skin surface react with DHA which results in tanning. Depending on several factors, the tan from DHA lasts from 3-10 days.