The Risks of Dyeing
A 2001 study from researchers at the University of Southern California discovered a relationship between the use of permanent hair dye and bladder cancer risk. They made a determination of those who are at highest risk from use of permanent dyes:
Women who use permanent hair dyes once a month for a year or more have twice the risk of bladder cancer.
Women who use permanent hair dyes at least monthly for fifteen years or more have three times the risk of bladder cancer.
Those who have worked as hair dressers or barbers for ten years or more have five times the risk of bladder cancer.
Darker hair dyes cause higher risk.
There was no association between semi-permanent or temporary hair dyes
International Journal of Cancer, February, 2001