In 2009, the International Agency on Cancer Research (IARC) classified occupational exposure to hair dyes a probable carcinogen (cancer causing substance) for hairdressers and barbers. This is due to the intensity and duration of exposure to hair dyes via inhalation or absorption through the skin. IARC is a part of the World Health Organisation which convenes international expert working groups to evaluate the evidence of the carcinogenicity of specific exposures.
Personal use of hair dyes is not classified as carcinogenic. People who colour their own hair are unlikely to have an increased risk of cancer, even if they have been colouring their hair regularly for many years. If you are still concerned, ensure that you colour your hair in a well ventilated room or salon, so as to minimise exposure to the fumes from hair dyes, and follow the manufacturers’ instructions.