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Dihydroxyacetone (DHA) is a simple three-carbon sugar. Applied to skin, it reacts with amino acids in the keratin of the stratum corneum in a Maillard reaction — the same non-enzymatic browning that colours toast, roast meat and beer — producing brown polymers called melanoidins.

Four consequences follow directly:

1. The colour develops over two to eight hours, because the reaction takes time.
2. It affects only the dead outer layer, so nothing penetrates to living tissue.
3. It fades in five to seven days as that layer sheds — the natural desquamation cycle.
4. It provides essentially no sun protection. A DHA tan has an SPF of roughly 3 at most. Sunscreen is still required.

Erythrulose is a related sugar used alongside DHA; it develops more slowly and gives a less orange tone, and blends of the two are common.

Self-tan failure is entirely an application problem, and the fixes are known:

1. Exfoliate the day before, not immediately before — see Body Scrub. Use an oil-free scrub; residue blocks the DHA.
2. Shave or wax 24 hours before, not after.
3. Moisturise the dry patches — knees, elbows, ankles, knuckles, heels. These absorb more DHA and go darkest.
4. Use a mitt. Palms take colour instantly and are the classic giveaway.
5. Long sweeping strokes, then buff outward at the wrists, ankles and hairline.
6. Let it dry fully before dressing; wear loose dark clothing.
7. Wait the stated time before showering, then avoid long hot baths, swimming and heavy exfoliation.
8. Moisturise daily afterwards — the tan lasts as long as the surface layer does.

Colour guides — the temporary tint in most products — show where you have been and wash off. They are the reason a modern self-tan is far easier than one from twenty years ago.

Cosmetic in the US — FDA primary category (15) suntan preparations — and DHA is regulated as a colour additive, which is the important part.

DHA is approved for external application to the human body only. FDA states explicitly that it is not approved for application to the area of the eye, the lips, or mucous membranes, and that its safety has not been established for exposure of those areas — a restriction that matters most for Spray Tan Solution booths.

A self-tanner with no sunscreen and any suntanning positioning must carry the 21 CFR 740.19 warning that it does not protect against sunburn.

In the EU, DHA is permitted in cosmetics; the SCCS has assessed it for both self-tanning products and spray applications, with conditions of use.

• It stains. Towels, sheets, light clothing and bathroom grout. Use dark towels and rinse the shower.
• It has a smell. The Maillard reaction produces characteristic volatile by-products, and no fragrance fully covers it. It is strongest at four to six hours.

Six to twelve months after opening, and DHA is genuinely unstable — it degrades faster in heat and at the wrong pH, and degraded DHA gives a weaker and more orange result.

Store it cool and dark, not in a bathroom and not in a beach bag. A product that has darkened in the bottle, thinned or developed a strong off smell has degraded and will give an uneven, orange result.

• Sun and Tanning — the shelf this sits on, and the rest of it.
• Cosmetics — what the label is legally allowed to say, and the full catalogue.
• The rest of Sunless — Self-Tanning Mousse · Self-Tanning Drops · Gradual Tanning Lotion · Tanning Water · Spray Tan Solution · Tan Remover.
• Leg Makeup — the wash-off alternative.
• Regulatory statements here follow the FDA and EU sources listed on Cosmetics, read 16 August 2026.