Scientists have called for more research into hair dyes and perming products to assess whether they contain a harmful substance which has been linked to causing cancer.
The comments come after researchers found that hairdressers had a high concentration of a type of carcinogenic chemical in their blood.
Toluidines are supposed to be banned from EU products, but the Swedish study found that hairdressers who apply light-colour permanent dyes or perform perms on their clients seem to be exposed to certain types of the compounds.
The paper, published in the journal Occupational and Environmental Medicine, examined blood samples from 295 female hairdressers, 32 users of hair dyes and 60 controls.
They found that concentrations of ortho-toluidine and meta-toluidine increased significantly with the weekly number of hair waving and permanent hair dyeing treatments.
Concentrations also tended to increase with the frequency of light-colour hair dyeing treatments, they said.
The authors said that hairdressers should protect themselves by wearing protective gloves and urged them to perform work where gloves cannot be worn, like cutting hair, before applying dye.
"Hairdressers who use light-colour permanent hair dyes, other permanent hair dyes and hair waving treatments seem to be exposed to o- and m-toluidine as indicated by associations with the number of treatments performed," the authors wrote.
"Analyses of hair waving and hair dye products should be performed to identify the possible sources of exposure to o- and m-toluidine.
"Hairdressers should protect themselves from dermal uptake by using protective gloves, and perform work where gloves cannot easily be worn (like cutting the hair) before dyeing."