To permanently colour the hair, hair dyes use a combination of the chemicals ammonia and hydrogen peroxide.
- The hydrogen peroxide works by bleaching out the natural colour and releasing oxygen, which allows chemical reactions to take place.
- The ammonia works by breaking down the outer cuticle around the hair shaft, allowing the other chemicals to enter the hair, where the colour development can take place.
These chemicals are quite harsh and can harden and thin the hair and they're also thought to irritate the skin, eyes and respiratory system. Another chemical that's been liked to severe allergic reactions is p-phenylenediamine (PPD), which works by reacting with other chemicals to develop the colour.
Around 4% of the trialists reported reactions to the dyes such as an itchy or stinging scalp, an itchy rash on the sides of the face and sensitivity to heat.
To avoid allergic or sensitive reactions, the packets advise you to perform a patch test 48 hours prior to application, even if you have used the product before, as it's possible for a reaction to develop after long-term use.
In the past, their has been some concern from health authorities that the chemicals in hair dye could lead to bladder and breast cancer, however in the Journal of the American Medical Association, there's no clear association between the two.
In the study, Canadian researchers conducted a review of the medical literature on varying forms of cancer and found there was no increased risk, including for people who dye their hair regularly.
The chemicals used in cosmetics are regulated by the National Chemicals Notification and Assessment Scheme (NICNAS), to ensure they remain at safe levels and are clearly labelled for consumer safety.