Para-phenylenediamine (PPD, MW 108 Da) is a primary aromatic amine used as an oxidative pigment precursor in permanent hair dyes. Its small molecular size allows it to penetrate the hair cortex, where it couples with couplers under hydrogen peroxide action to form large colored molecules. It is also one of the most frequent contact allergens in cosmetics — through T-lymphocyte sensitization. Hairswiss analyzes its chemistry, coloring mechanism and documented dermatological risks.