There are approx. 100 different colorants that can be used in the composition of hair dyes, among which several substances are known allergens.
One of these, PPD (p-phenylenediamine) has been used to dye hair for the last hundred years and is known to be extremely allergenic. Some colorants are associated with PPD and these are either proven to be allergenic or likely to be.
In addition to PPD is the most frequently used substances: toluene-2,5-diamine, resorcinol, 4-amino-2-hydroxytoluene, p-aminophenol. Toluene-2,5-diamine often replaces PPD in hair dyes in the Scandinavian market, but the substance has the same allergenic properties as PPD.
PPD substances are used in hair dyes in different amounts, depending on the desired shade, among other things. PPD is typically used in the dark and red colours, but simply looking at the color tone is not a way to determine if a hair dye contains allergens.
There are different types of hair dye. There are the permanent hair dyes in which the dye penetrates the hair shaft and cannot be rinsed out. In addition, there are colours which tone the hair for a shorter or longer period, but which can be washed out. PPD is one of the dyes typically used in permanent hair dyes, but PPD-like substances are also used in hair dyes that can be rinsed out.
Herbal extracts are also used as hair dye. The best known is henna. Henna rarely causes allergic skin reactions. But henna is sometimes mixed with other hair dye substances, for example PPD or picramic acid, but still marketed as henna.
