The Chemistry of Fake Tan

photo from: https://www.superdrug.com/blog/beauty-manuals/the-ultimate-guide-to-applying-fake-tan

As discussed in a previous blog post (https://glamsci.blog/2019/04/23/how-does-sunscreen-protect-our-skin/), prolonged exposure to harmful UV rays can be very dangerous, but even with this knowledge, many people still desire sun-kissed skin. A safer alternative to sunbathing or using a sunbed is fake tan, a cosmetic product that uses chemicals to temporarily darken the skin, but how does it work?

Fake tan commonly contains a simple organic compound called dihydroxyacetone (DHA), which is responsible for the brown colour the skin adopts after application. The outermost layer of the skins epidermis contains many amino acids and proteins that make up the skin, and these react with the DHA in fake tan in a number of complex steps to form a colourless product. This then undergoes a number of further reversible rearrangement reactions with amino acids to produce complex, large visible-absorbing brown polymers also known as melanoidins (figure 1).1,2

Figure 1.

DHA is the active ingredient in almost all fake tan products and is sometimes accompanied by another chemical known as erythrulose. This molecule has a similar structure to DHA and acts in a similar way, reacting with amino acids in the skin but at a much slower rate.3 Both DHA and erythrulose do not penetrate deeper than the first layer of the skin, but are both thought to react with amino acids through pathways that involve free radicals. Free radicals are extremely reactive molecules, and it has been suggested that after using fake tanning products, users should refrain from UV exposure for 24 hours as the risk of free radical injury is increased.2

Fake tan products containing DHA are usually maintained between pH 4 and 5 which keeps them stable. Decreasing the pH of fake tan products is known to give a more natural looking tan, and more recently additives which allow the pH of fake tan formulations to be lowered without irritating the skin have been developed.4

It has also been suggested that treating the skin with amino acids before fake tan application could allow enhanced colouring. One proposal is to apply sulfur-containing amino acids onto the skin beforehand which creates more binding sites for the active DHA ingredient to react with.5

References

  1. B. C. Nguyen and I. E. Kochevar, J. Invest. Dermatol., 2003, 120, 655–661.
  2. K. Jung, M. Seifert, T. Herrling and J. Fuchs, Spectrochim. Acta – Part A Mol. Biomol. Spectrosc., 2008, 69, 1423–1428.
  3. C. G. Burkhart and C. N. Burkhart, Open Dermatol. J., 2009, 3, 42–43.
  4. G. Pantini, R. Ingoglia, F. Brunetta and A. Brunetta, Int. J. Cosmet. Sci., 2007, 29, 201–209.
  5. B. C. Nguyen and I. E. Kochevar, Br. J. Dermatol., 2003, 149, 332–340.

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