How does fake tan work, and why does it sometimes smell ‘biscuity’?

Inna Rabotyagina/Pexels Real tans come from UV-triggered pigment production in living skin; that same UV exposure increases the risk of skin cancer.

Bronzers are simply coloured coatings that sit atop the skin and can be washed away.

How, then, to create a fake tan that stays put, without exposing people to UV light and skin cancer risk? The trick is not to get brown pigment into the skin directly.

Brown pigments are usually large molecules, which do not penetrate the skin barrier easily.

Instead, fake tan products smuggle in something much smaller, then generate the pigment chemically once it is already inside the outer skin layer.

How to smuggle in a molecule The outermost layer of skin, the stratum corneum, is dead skin cells embedded in lipids (fats).

While it is an excellent barrier, some small molecules can still get through.

The active ingredient in most fake tan products is dihydroxyacetone, or DHA – a tiny sugar molecule found naturally in our bodies.

It’s closely related to glycerol (or glycerin), the sweet, syrupy compound found in foods, cough syrups and skincare products.

Glycerol is in moisturisers and many personal care products; it helps keep skin hydrated.

It’s also small enough to move through the outer skin barrier reasonably well.

Just like glycerol, DHA can diffuse into the outermost layers of skin.

But instead of helping skin retain water, it reacts chemically with free amino acids and proteins in the stratum corneum, forming brown pigments.

So unlike bronzers or makeup, DHA does not simply sit on top of the skin.