Hair dyeing

Permanent colouring

These products provide effectively permanent coloration and represent at least 80% of the hair colouring market. Hair dyeing with permanent colouring provides both the most effective hair coloration and maximum flexibility. In this system, uncoloured precursors diffuse into the hair and undergo oxidation reactions (with hydrogen peroxide being the usual oxidant) to produce the desired colour in situ. These colorants usually last at least 24 shampoos. This category is also called 'oxidative colouring' and the chemistry is similar to that used in colour photography.

Toxicology

The safety of hair dye ingredients of all classes is thoroughly reviewed in the US by the Cosmetic Ingredient Review (CIR) panel, and the findings published in the International Journal of Toxicology. Similar regulatory bodies exist in all developed countries.The occurrence of dermatitis or other allergic responses to the use of hair dye products is quite rare, however many countries have regulations requiring users to carry out prophetic patch tests before dyeing. A cautionary statement may also be mandated.