PPD in Hair Dye: What It Actually Is and Why It's There
by Wooly Team
PPD — para-phenylenediamine — is one of the most common ingredients in hair colour and one of the most misunderstood. It is also the ingredient behind most hair-colour allergic reactions, which makes it worth understanding properly rather than avoiding by reputation.
What is PPD?
On its own, PPD is a colourless compound. It is not a dye yet. Combined with an alkalising agent and an oxidising developer it reacts, and those reactions build the larger coloured molecules that sit inside the hair shaft.
That is the whole mechanism of oxidative hair colour: the pigment is not poured onto the hair, it is assembled inside it. It is also why the result survives washing in a way that a surface pigment cannot. Because the mechanism works so well, PPD and related dye intermediates appear across the great majority of permanent and semi-permanent hair colour — ammonia-based and ammonia-free alike.
Ammonia-free does not mean PPD-free
This is the single most common mix-up, and it is worth being precise about. Ammonia and PPD do two entirely different jobs.
Ammonia — or its usual substitute, ethanolamine — is the alkalising agent. Its job is to raise pH so the cuticle lifts and the colour precursors can get in. PPD is the colourant. Its job is to form the pigment once they are in there.
Take the ammonia out and the colourant is untouched, doing exactly what it did before. So switching to an ammonia-free product does not reduce PPD-related allergy risk at all. Ammonia-free is a real and worthwhile change — less cuticle disruption, less scalp irritation, no sharp smell — but it is a change to a different ingredient. Our complete guide to ammonia-free hair colour goes through what the term does and does not cover.
Why PPD causes allergic reactions
PPD is a well-documented contact allergen. The reaction it provokes is a delayed one: it does not happen on contact, but hours to days afterwards, which is what makes it easy to miss and easy to misattribute to something else.
Sensitisation also builds over time. Someone can use the same colour product without incident for years and then react, because the immune response develops with repeated exposure rather than being fixed from the first use.
That delayed timeline is the reason patch testing works the way it does, and the reason the waiting period is 48 hours rather than twenty minutes. We have set out the step-by-step patch test method separately.
Is Wooly's formula PPD-free?
No. Wooly Hair Colour Shampoo lists p-Phenylenediamine and Resorcinol among its ingredients. It is genuinely ammonia-free, and that claim is accurate — but ammonia-free and PPD-free are two different claims, and only the first one is being made.
In practice that means a patch test 48 hours before first use is mandatory, and the product is not recommended for sensitive or allergy-prone skin. Wooly Beard Touch-Up is a different case entirely: it is a pigment stick containing neither PPD nor Resorcinol, because it colours the surface of the hair rather than reacting inside it.
The bottom line
No mainstream wash-resistant hair colour avoids PPD altogether. Treating "PPD-free" as the thing to shop for will mostly lead you to products that either do not last or do not cover. The more useful safety question is not does this contain ammonia, and not even does this contain PPD — it is have I patch tested this specific product.
Wooly Hair Colour Shampoo is ammonia-free and declares its colourants openly — p-Phenylenediamine and Resorcinol are both on the pack. See the full ingredient list →