Guide 01 / Chemistry
DHA: the molecule that turns your skin brown.
Every modern self-tan on the market works on the same active ingredient — dihydroxyacetone, or DHA. It is the most important variable on a self-tan label and the least well-explained on it. Here is what it is, what it does, and how to read the percentage.
Active
C3H6O3
Dihydroxyacetone (DHA)
CAS 96-26-4 · Molecular mass 90.08 g/mol
What DHA actually does
Dihydroxyacetone is a small three-carbon sugar. When applied to skin it migrates into the outermost layer — the stratum corneum, made of dead keratin-rich cells — and reacts with free amino acids and proteins in those cells. The reaction is a non-enzymatic Maillard reaction: the same family of chemistry that browns toast, sears steak, and gives baked biscuits their colour.
The end products are melanoidins — large brown nitrogen-containing pigments. They are not melanin (the pigment your body produces in response to UV), but they sit in the same outer layer of skin and look broadly similar. Because the stratum corneum sheds naturally over 5 to 7 days, a DHA tan fades on that timeline regardless of the product. There is no chemical that bonds the colour into living skin.
How the percentage maps to results
DHA concentration is the single largest determinant of how deep a tan develops and how quickly. UK self-tan products typically fall into four bands:
| DHA % | Result band | Typical use |
|---|---|---|
| 2–3% | Gradual / very subtle | Daily moisturiser tans, first-timers, fair skin |
| 4–6% | Light to medium | The most common consumer range. Buildable. |
| 7–10% | Dark | Express / overnight darks, more visible after one application |
| 11–15% | Ultra-dark / professional | Salon spray solutions, ultra-dark home products |
A product's "develop time" is partly determined by DHA concentration. Higher concentrations react faster — which is why a 1-hour express tan typically sits above 7% DHA, and why a gradual moisturiser at 2–3% needs repeat applications across several days to build comparable depth.
Why some tans smell biscuity
The Maillard reaction produces volatile aroma compounds — pyrazines, furans, and other nitrogen/oxygen heterocycles — as side products. These are the same chemical families responsible for the smell of bread crust, malt, and roasted nuts. There is no way to fully prevent the smell of a DHA tan because it is the smell of the reaction itself. Brands mitigate it through encapsulation, fragrance overlays, or fast-development formats that produce fewer volatile byproducts before rinse-off.
Anyone who has ever used St. Tropez Classic Mousse will recognise the "biscuit" note — that is the Maillard reaction in real time. Fragrance-free formulas (Bondi Sands Pure, Isle of Paradise Drops) do not eliminate the smell, only the perfume on top of it.
Erythrulose: DHA's slower cousin
Many modern formulations pair DHA with erythrulose (C4H8O4), a four-carbon sugar that reacts via the same Maillard chemistry but more slowly — typically 24 to 48 hours to peak. The combination produces a more even, longer-lasting result because the two pigment-producing reactions overlap rather than peaking and fading together. Look for both on the INCI list of any product marketed as "long lasting" or "two-week wear."
What the percentage doesn't tell you
DHA concentration is not the whole story. Identical DHA percentages can produce noticeably different results depending on:
- pH. DHA is most stable and most reactive at pH 4–5. Products formulated outside that range can either underperform (alkaline) or feel acidic (overly acidic).
- Vehicle. Water-based tans (Bondi Sands Pure Foaming Water, St. Tropez Purity) deliver DHA in a thin layer that absorbs fast; mousses and lotions hold DHA against the skin longer, giving deeper reaction.
- Skin condition. Dry, alkaline, or freshly-exfoliated skin reacts more intensely with DHA. Hydrated, balanced skin produces a more even result. This is why patch-test results are not always representative of full-body results.
- Stabilisers. DHA degrades in the bottle over time, especially after opening. Antioxidants (Vitamin E, tocopherol), pH buffers, and chelating agents (EDTA, sodium phytate) extend shelf life.
Reading an INCI list, practically
Self-tan ingredients in the EU and UK are listed by INCI name in descending order of concentration (above ~1%). For a typical mousse, the first 3–5 listings will be water, the surfactant system, DHA, and one or two humectants (glycerin, propylene glycol). DHA's position in the list approximately indicates its concentration — a DHA listing in slot 2 or 3 generally indicates a 6%+ formulation; a listing in slot 5 or lower suggests a gradual or light formula.
The bottom line
DHA is the only active ingredient that produces a self-tan colour. Everything else on the label — fragrance, oils, hyaluronic acid, vitamin E, niacinamide — is either there to help DHA reach skin evenly, to extend its shelf life, to mitigate its smell, or to make the product feel pleasant. When choosing a self-tan, the DHA percentage and the format are doing the work. The skincare ingredients are decoration.
All guides