Dihydroxyacetone

Dihydroxyacetone, nearly always shortened to DHA on an ingredients list, is the colouring agent behind almost every self-tanning product sold today. Chemically it is a triose: the simplest possible ketone sugar, built from just three carbon atoms. Commercially it is most often produced by fermenting glycerine, which is why you often see it described as sugar-derived.

In the bottle, DHA is a white, faintly sweet-smelling crystalline powder that dissolves easily in water, and it goes onto skin colourless. The brown tone only appears over the hours that follow, which is why many formulas carry a temporary, washable guide colour so you can see where you have applied.

It has been used in cosmetics since the 1950s. In the UK and EU it is regulated as a restricted cosmetic ingredient with a legal maximum in self-tanning products; in the US it is separately classified as a colour additive for externally applied cosmetics. Either way it is meant for the body and face, not the lip area, the eye area or mucous membranes. Self-tanners sold in the UK generally contain roughly 1% to 10% DHA — 10% is the legal maximum here. The lower end sits in gradual tanning moisturisers and daily lotions, while mousses and darker-developing products sit towards the top of that range.

You will find it in mousses, lotions, drops, waters, mists, wipes and professional spray-tan solutions, often alongside erythrulose, a related sugar that develops more slowly and helps soften the final tone. DHA is not a sunscreen: the colour it creates offers no meaningful protection from UV.

The colour comes from a Maillard reaction — the same family of non-enzymatic browning that gives bread crust and roasted coffee their colour. DHA's carbonyl group reacts with free amino groups on the amino acids and proteins sitting in the stratum corneum, the outermost layer of flattened, already-dead skin cells. That first step sets off a cascade ending in melanoidins: large, brown, polymer-like pigments that lodge in those surface cells. Laboratory work indicates basic amino acids such as lysine, arginine and histidine react particularly readily, and that lysine side chains within skin proteins contribute meaningfully to how deep the colour goes.

Because the pigment forms in cells already on their way off the surface, the effect is inherently temporary. It fades as those cells shed naturally, typically over five to ten days, and disappears faster wherever skin is exfoliated or rubbed more.

Several conditions steer the outcome. Research shows the reaction runs faster at close-to-neutral pH than at the skin's more acidic surface, and that moisture matters in a non-obvious way: colour develops best at moderate humidity, while very dry skin and a soaking, occluded surface both hold it back. Thicker, rougher patches of stratum corneum carry more reactive material, which is why elbows, knees and ankles pick up more colour.

• Exfoliate 24 hours ahead, not on the day — most patchy results trace back to preparation rather than technique. Removing loose surface cells first means the colour lands on an even canvas and lasts longer before it starts to fade unevenly.
• Wax or shave the day before, so open follicles have time to settle and do not take up extra colour as speckles.
• Buffer the dry spots. Work a light, oil-free moisturiser into elbows, knees, ankles, hands and feet just before you apply — thicker skin grabs more pigment.
• Apply clean, dry and with a mitt. Skip deodorant, perfume and body oils, let skin cool down after a shower, work section by section in long sweeping strokes, and wash your palms straight afterwards.
• Give it time. Follow the stated development window — often four to eight hours — and avoid sweating, swimming and tight clothing until you have rinsed.
• Build gradually. Reapplying every few days deepens the tone far more predictably than one heavy coat.
• Moisturise daily afterwards to keep the fade even, and still apply sunscreen.