para-Phenylenediamine (PPD) and other related members of the aromatic amine family have been the main agents used in permanent hair dyes for >100 years. PPD is used in ≥70% of hair dyes worldwide [1], with most of the other dyes containing compounds that are chemically related to PPD. With its low molecular weight (108 Da) and ability to penetrate the hair shaft and follicle, its strong protein-binding capacity, and its rapid polymerization in the presence of a coupler and an oxidizing agent, PPD is an ideal chemical for hair dyeing. Almost all permanent hair dyes belong to the same aromatic amine family, in which there is likely to be a significant degree of immunological cross-reactivity [2]. At present, there is no effective alternative to the use of PPD or other chemically similar aromatic amines in permanent hair dyes. PPD is also used for dyeing facial hair (e.g. eyebrows, eyelashes, and beard hair) and in henna temporary tattoos, which are painted on the skin; the use of PPD for some of these functions is illegal in the European Union, although some PPD derivatives are allowed.