- ammonia
- peroxide
- p-phenylenediamine (PPD)
- diaminobenzene
- toluene-2,5-diamine
- resorcinol
Between them they create risks which must be carefully evaluated, and patch tested, before every hair dye application.
Here’s a list of the seven most toxic common chemicals found in hair dye : https://www.theepochtimes.com/avoid-these-7-toxic-chemicals-found-in-most-hair-dyes_2040018.html
Sadly, due to hair dye deaths and hair dye reactions all these ingredients are all potentially harmful and ought to be regularly highlighted as a health hazard.
Chemicals in hair dye mean you MUST patch test before every hair dye application to stay safe.