Permanent hair dyes, also called oxidative dyes, work with the help of hydrogen peroxide, which causes a chemical reaction between so-called 'intermediates' - the aromatic amines - and dye 'couplers'.
Many of the aromatic amines called toluidines have been implicated in causing cancer, but ortho-toluidine (o-toluidine) has actually been confirmed as a carcinogenic.
Aromatic amines have been banned in cosmetic products in the European Union but a new study from Lund University in Sweden found that hairdressers may still be exposed to these carcinogens.
For the new study, researchers looked at o-toluidine levels in the blood of participants, along with levels of seven other aromatic amines, according to 'medicaldaily.com'.
Specifically, they looked at structures in the blood called hemoglobin adducts, which gave them a sense of long-term exposure to the carcinogen, as they showed how many molecules of hemoglobin had attached to molecules of the toluidines.
Testing this method on 295 female hairdressers, 32 regular users of hair dye, and 60 people who hadn't used any products in a year, they found that hairdressers were indeed exposing themselves to o-toluidine and meta-toluidine (m-toluidine).
The hairdressers who performed more weekly hair dye or hair waving treatments tended to have higher concentrations of the carcinogens.
Researchers said they were able to narrow down the source of o-toluidine to "light-colour permanent hair dyes and hair waving".
The paper was published in the journal Occupational and Environmental Medicine.