Much of the evidence linking hair dyes with bladder cancer comes from studies of hairdressers. In seven of 10 populations studied (from the US, Norway, Sweden, Finland, Denmark, Japan), scientists found elevated incidence of bladder cancer among hairdressers, barbers, beauticians and cosmetologists exposed to hair dyes — 40 percent higher, on average, than population-wide risks. Hair dye exposure was also linked to bladder cancer in seven of 12 case-control studies focused specifically on occupational history among bladder cancer victims (Gago-Dominguez et al. 2001).
The International Agency for Research on Cancer found that “occupation as a hairdresser or barber entails exposures that are probably carcinogenic” (IARC 1993), and a recent study by scientists from the University of Southern California’s School of Medicine shows that hairdressers and barbers with more than 10 years on the job face a five-fold increase in bladder cancer risk compared to people not exposed to hair dye (Gago-Dominguez et al. 2001).
Studies in people
Most of the studies looking at whether hair dye products increase the risk of cancer have focused on certain cancers such as non-Hodgkin lymphoma, leukemia, bladder cancer, and breast cancer. These studies have looked at 2 groups of people:
People who use hair dyes regularly
People who may be exposed to them at work
Most studies of people exposed to hair dyes at work, such as hairdressers and barbers, have found a small but fairly consistent increased risk of bladder cancer. However, studies looking at people who have their hair dyed have not found a consistent increase in bladder cancer risk.
Studies looking at a possible link between personal hair dye use and the risk of blood-related cancers such as leukemia and lymphoma have had mixed results. For example, some studies have found an increased risk of certain types of non-Hodgkin lymphoma (but not others) in women who use hair dyes, especially if they began use before 1980 and/or use darker colors. The same types of results have been found in some studies of leukemia risk. However, other studies have not found an increased risk. If there is an effect of hair dye use on blood-related cancers, it is likely to be small. (International Agency for Research on Cancer,Volume 57)