p-Phenylenediamine (PPD)

What it is

An aromatic amine that, mixed with peroxide and a coupler, forms dye inside the fibre. It is used in permanent dyes, especially black and brown. It is a cosmetic ingredient in the US, EU and UK, allowed up to regulated on-head concentrations.

How it works

Oxidised PPD couples with other dye components inside the cortex to form large coloured molecules. The allergy is a delayed-type contact sensitisation. A review describes allergic sensitisation, acute dermatitis and severe facial swelling, so avoidance is the only reliable prevention once you are sensitised. Thyssen 2008

What the studies show

• A review of patch-test data found median PPD sensitisation of 4.3% in Asia, 4% in Europe and 6.2% in North America among dermatitis patients, with a plateau of 2-6% in Europe. Thyssen 2008
• "PPD-free" dyes can still contain related allergens, so label reading is not enough if you are allergic.

How it's used in studies

Home kits and salon dyes use concentrations set by regulators. Box labels recommend a 48-hour patch test.

Side effects and safety

Serious in sensitised people: severe facial swelling, blistering and, rarely, systemic reactions. Cross-reaction with related para-amino dyes is described. Avoid black henna, which often contains high PPD. Stop using it if you previously reacted, and see a dermatologist for patch testing.

Bottom line

Fine for most people with a patch test, but any itch or burn on a previous dye means stop. If you are allergic, you need patch testing, not just label reading. We are confident in the hazard. See bleach and colour damage.