The Health Dangers of Hair Dyes

Bladder cancer among hairdressers: a meta-analysis.

 http://www.ncbi.nlm.nih.gov/pubmed/20447989

In summary, our results showed an increased and statistically significant risk for bladder cancer among hairdressers, in particular for hairdressers in jobs held > or = 10 years. Residual confounding by smoking cannot be totally ruled out. Here's the link above for article.