Hair dye ingredients explained: PPD, toluene-2,5-diamine, resorcinol and the warnings on the box
A permanent hair colour is a small chemistry set: dye precursors, couplers, an alkaline agent and hydrogen peroxide. What names like p-Phenylenediamine, Toluene-2,5-Diamine and Resorcinol mean on the box, and why the warnings are there.
Updated: 31 October 2026 · Reviewed by: Kamil Pyziak
How permanent hair colour works
Most box dyes and salon colours that promise lasting, grey-covering results are oxidative dyes. The colour tube does not contain finished dye. It contains small, almost colourless molecules that become coloured only when they react with an oxidising agent – which is why you mix two components just before use: a colour cream or gel and a developer.
An alkaline agent swells the hair so the small molecules can get inside. The developer, usually hydrogen peroxide, starts a reaction in which these molecules join up into larger coloured molecules. Once formed inside the hair, they are too large to wash out easily. The same peroxide also lightens the natural pigment to some degree, which is how a dye can go lighter than your starting shade.
Precursors: PPD and toluene-2,5-diamine
The molecules that drive the reaction are called precursors or primary intermediates. The best known is p-Phenylenediamine, often shortened to PPD, used especially for dark and natural shades. Its close relative Toluene-2,5-Diamine (also as Toluene-2,5-Diamine Sulfate) is very widely used in products sold in Europe. Other names in this group include p-Aminophenol, Hydroxyethyl-p-Phenylenediamine Sulfate and 2-Methoxymethyl-p-Phenylenediamine.
These substances are known skin sensitisers: in some people, repeated contact can lead to a contact allergy. People who are allergic to one of them may also react to chemically related ones. This is the main reason why oxidative hair dyes are among the most strictly regulated cosmetic products and carry long warnings.
Couplers: resorcinol and friends
Precursors on their own would give a limited range of colours. Couplers react with them and fine-tune the shade. Typical INCI names are Resorcinol, 2-Methylresorcinol, m-Aminophenol, 4-Amino-2-Hydroxytoluene, 2,4-Diaminophenoxyethanol HCl and 1-Naphthol. A single shade usually combines several precursors and couplers, so two shades from the same brand can have quite different ingredient lists.
Resorcinol is one of the most common couplers, and products that contain it carry a specific statement on the label. In the EU, hair dye substances are allowed only if they are listed in the annexes of the Cosmetics Regulation, with maximum concentrations that refer to the mixture applied to the hair after mixing.
Developer, ammonia and “ammonia-free”
The developer is a stabilised solution or cream of Hydrogen Peroxide. Its strength is given either as a percentage or in “volumes”: 3% corresponds to about 10 volumes, 6% to 20, 9% to 30 and 12% to 40. Higher strengths lighten more. Box dyes come with a matched developer; professional products let the hairdresser choose.
The alkaline agent is traditionally Ammonia, which has a strong smell. “Ammonia-free” dyes typically use Ethanolamine (monoethanolamine) instead. This changes the smell and the way the product works, but an ammonia-free permanent dye still relies on the same kind of precursors and couplers. You will also see helpers such as Sodium Sulfite or Ascorbic Acid, which protect the dye molecules from oxidising in the tube, and chelating agents such as Disodium EDTA.
Semi-permanent dyes, direct dyes and henna
Non-oxidative or direct dyes are already coloured. They sit mainly on and just under the surface of the hair and fade over several washes. Their INCI names look different: HC Blue No. 2, HC Yellow No. 2, Basic Red 51, Basic Blue 99 and similar. Be aware that “demi-permanent” or “tone-on-tone” products can still be oxidative dyes with a milder developer, so the ingredient list matters more than the name on the front.
Henna comes from the leaves of Lawsonia inermis and gives orange-red tones. Products sold as “black henna” for temporary skin tattoos, often outside the EU, have been found to contain PPD. That is why EU hair dye warnings mention black henna tattoos.
The warnings on the box
In the EU, hair dyes containing these substances must carry mandatory warnings. They include statements that hair colorants can cause severe allergic reactions, that the instructions must be read and followed, that the product is not intended for persons under the age of 16, and that temporary black henna tattoos may increase the risk of allergy. They also list situations in which you should not colour your hair, for example after a previous reaction to a hair dye or to a black henna tattoo, or when the scalp is irritated or damaged. Labels also say which dye substances the product contains and tell you to wear suitable gloves.
Many manufacturers instruct users to carry out an allergy alert test, typically 48 hours before every colouring, even with a familiar product. Follow the leaflet of the specific product exactly. If you have any questions about your own risk, or if you notice a reaction during or after colouring, contact a doctor or pharmacist.
What the label does not tell you
The INCI list shows which precursors and couplers are in a shade, but not their amounts or the colour that will form on your hair. A claim such as “PPD-free” only says that one substance is absent; the product may contain toluene-2,5-diamine or other precursors. Words like “natural” or “gentle” on the box say nothing about allergy potential.
Salon colours are mixed from professional products whose boxes you may never see. If ingredients matter to you, ask your hairdresser for the product name or the INCI list before the appointment.