DHA is a colourless chemical (it is derived from glycerine) that interacts with the amino acids in dead skin cells and oxygen to produce a brown colour change. It is like a cut apple turning brown when it is exposed to the air. DHA is not absorbed through the skin in the the body and it has no known toxicity. When the spray tan solution goes on you will instantly see colour, this is not the tan, this is the bronzer which is added to the solution so the applicator can see where the solution has gone. This is going to wash off when you have your first shower. The colour will continue to develop on your skin for up to 12 hours.