P-phenylenediamine: the usual suspect
The practice of colouring hair dates back to before the Middle Ages. The main agents used of old were natural vegetable extracts, but those are hardly used any more, whether in home hair dye kits or in hair dyes applied by professionals. For the last 100 years or so, p-phenylenediamine (PPD) has been the main agent used in permanent and semi-permanent hair dyes. It has a number of properties that make it a far more effective hair dye than other agents. However, it is also an important allergen.
Dermatology in practice 2009; 17(2): 8–10
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