For more than 100 years para-phenylenediamine (PPD) and other related members of the aromatic amine family have been the main agents used in permanent hair dyes, and more than two thirds of hair dyes currently contain PPD. This compound is an effective hair dye owing to its low molecular weight, its ability to penetrate the hair shaft and follicle, its strong protein binding capacity, and its rapid polymerisation in the presence of a coupler (a kind of catalyst) and an oxidising agent. However these properties also make PPD an ideal contact allergen and, indeed, it is among the most potent.1

During the 20th century allergic reactions to PPD became such a serious problem that it was banned from hair dyes in Germany, France, and Sweden.2 Current European Union legislation allows PPD to comprise up to 6% of the constituents of hair dyes on the consumer market (3% when added to the oxidising solution required to develop the colour). …