p-Phenylenediamine (PPD)

What it does

The main colour-forming ingredient in most permanent dark hair dyes. Once mixed with the peroxide developer it reacts inside the hair with 'coupler' ingredients to build the final shade, which is why the colour survives washing.

Benefits

Gives deep, long-lasting dark shades and reliable grey coverage, which is why it is in so many permanent colours.

Concerns & Risks

One of the strongest skin allergens in everyday use. About 4–6% of patients tested in dermatology clinics react to it, and about 1.5% of the general population (mostly European data); in Indian clinic studies it is consistently among the most common allergens. Once you are allergic, an ordinary 30-minute dye job is enough to set off a reaction: itching and eczema along the hairline, ears and neck, and sometimes severe swelling of the scalp, face and eyelids that can need hospital care. Rarely it causes hives and breathing difficulty. In Indian users it is also linked to dark patches on the forehead, temples and ears. It cross-reacts with toluene-2,5-diamine and other hair-dye amines, and in some people with textile dyes, benzocaine-type local anaesthetics and sulfonamide medicines. Black henna tattoos, which are henna mixed with PPD, are a major cause of first sensitisation and of the most severe later reactions. It is poisonous if swallowed.