p-Phenylenediamine

At a glance

Function: Oxidative dye precursor
Origin: Synthetic
Product areas: Hair colour

What it is

p-Phenylenediamine, PPD for short, is the best-known ingredient in hair colour and at the same time its most important allergen. It is a dye precursor: colourless in itself, under the action of hydrogen peroxide it forms, together with the couplers, the colour pigments that are then locked inside the hair.

What matters is when the substance acts as an allergen. Fully reacted PPD is not a sensitiser. The critical state is the intermediate one — the partially oxidised molecule during the development time. That explains why with this substance the application is central and not the result, and why rinsing thoroughly counts for more than anything printed on the pack.

Assessment

Result: Increased attention
Confidence: medium
Evidence: A — very well established
Applies to: Cosmetics, external use on hair and scalp; here one hair colour, mixed, applied and rinsed out after the development time; occupational exposure on the hands is a route of its own and is expressly included here; concentration in the product not documented

Why

Why evidence level A appears here for the first time: on no other substance in this lexicon is the evidence base this broad. A population-based study in five European countries interviewed 10,425 people and patch tested 2,739 of them. The result: 0.8 per cent contact allergy to PPD. On top of that come systematic reviews and decades of test series from dermatology clinics.
In the clinics the figures are far higher — around four per cent positive results in Europe among the eczema patients examined, over six in North America. That is not a contradiction but a question of the group: anyone who comes to a dermatology clinic already has a problem. The 0.8 per cent describes the population, the four per cent a pre-selected group.
The most surprising finding of the same population study: no statistically significant association was found between hair colour use and PPD allergy. A different one was clear, however. Among people with black henna tattoos the allergy rate was 3.2 per cent against 0.6 per cent in the rest. Such tattoos contain PPD in unknown, often high amounts and in unreacted form — that is the most effective known way of sensitising a person to PPD.
The consequence only shows years later. A case series describes seven people who, on average 6.2 years after a black henna tattoo, developed allergic eczema when colouring their hair. Five of them also reacted to other dyes. Something practical follows for advice at the chair: the question about earlier henna tattoos is more revealing than the question about earlier colouring.
For staff, PPD is an occupational risk and is expressly listed in the specialist literature as an occupational disease in hairdressing. The route is skin contact on the hands while mixing and applying, not the client's scalp. That is the same pattern as with the persulphates, only through the skin instead of through the air.

When the assessment differs

Known PPD allergy: then the product is unsuitable, without gradation. Reactions can be severe, and with this substance that is not a figure of speech.
A previous black henna tattoo: the single most important risk factor. It often lies years back and is rarely mentioned unprompted.
Cross-reactions: anyone reacting to PPD frequently also reacts to related substances — toluene-2,5-diamine, p-aminophenol, m-aminophenol and further aromatic amines. Switching to another colour from the same substance class does not reliably help.
Salon staff: daily contact while mixing and applying, over years. Gloves are not a formality here.
Irritated or broken scalp skin: through a disturbed barrier more of the substance reaches the skin, and that is precisely what sensitisation depends on.

Open questions

Concentration in the specific product: not available.
Share of unreacted PPD after the development time: not determinable. It decides the actual exposure and depends on development time, amount of peroxide and how thoroughly the product is rinsed out.
The self-test before colouring recommended on the pack: the dermatological literature discusses whether such a test can itself sensitise. That question is not evaluated here, so it is neither recommended nor advised against.
Influence of the hair colour of the tested population on the regional differences: raised in the literature, not evaluated here.
Regulatory status: not checked; statements on use and maximum levels are not quoted second-hand.

Sources for this assessment

Applies to: Dye precursors and couplers

2,4-Diaminophenoxyethanol HCl — Oxidative dye precursor
2,7-Naphthalenediol — Coupler for oxidative hair colour
2-Methylresorcinol — Coupler for oxidative hair colour
4-Chlororesorcinol — Coupler for oxidative hair colour

Further substances from the same group for which a long-form assessment already exists.

Recorded in the following products

This list documents the products in which the ingredient was declared at the time of recording. It is not a statement about those products and not a recommendation. Manufacturers change formulations; the ingredient list on the current packaging is always authoritative.

OCS Colour Gel

Sources documenting its occurrence

Manual OCS Salon Manual Colour 2026 Professional manual supplied for this project