These days we’re all aware that suntans give you both cancer and wrinkles, so fake tan is the colour du jour. This miracle-in-a-can stains the skin brown through an interesting chemical reaction. The outer layer of your skin, made up of dead skin cells, is permanently coloured. The tan wears away as the skin cells come off. Here’s how it works, and whether it’s safe!
The Chemistry of Fake Tan
Fake tan products you find in stores contain 2-5% dihydroxyacetone, which looks like this:
It starts off colourless, but it reacts with amino acids (particularly arginine, lysine and histidine) in the skin to form a variety of brown compounds called melanoidins.
It’s actually the same chemical reaction as the one responsible for making food like bread and meat turn brown and delicious when cooked. It’s called the Maillard reaction. For the really intense chem nerds, it proceeds like this with DHA (I got a bit lazy, sorry for the shortcuts):
This forms covalent bonds, which means the skin is permanently stained – water, soap and moisturiser won’t wash it off. The skin starts devloping the tanned colour after 2-3 hours, and the reaction continues for the next 1-3 days. The reaction occurs best at moderately low acidic pHs (3-6), so fake tans tend to come in this skin-friendly range. The extent of the reaction is also influenced by the amount of water around.
Application and Aftercare
DHA only penetrates the very top layer of skin (the stratum corneum), which you may know is dead skin cells. This is why fake tans can’t last longer than about a week – that’s about how long it takes for the stained skin to wear off (that’s why if you look up fake tans that claim to last longer than a week or two, you’ll find tons of grumpy reviews – the skin sheds at a similar rate no matter what product you use!).
This explains all the advice given for making your tan look good and wear off evenly: