Originally published May 12, 2008. Updated November 19, 2013.
An estimated 66 to 74 percent of women in the U.S. color their hair, so the issue of whether this practice can be harmful is an important one. At this point, the answer to your question is no. I’ve seen no studies that demonstrate conclusively that using hair dye leads to increased risks of cancer or other diseases. The strongest evidence we have for such a link comes from epidemiological studies that found an increased risk of bladder cancer among hairdressers and barbers with high occupational exposure to these products. But even here the evidence is not conclusive. According to a review published in January, 2013, in the Journal of Clinical and Aesthetic Dermatology, the bulk of available data does not strongly support an association between hair dye use and bladder cancer among individuals in these professions. However, based on results of a previous study that found a significant relationship between hair-dye use and bladder cancer, the International Agency for Research on Cancer (IARC) concluded in 2008 that working regularly with hair dyes in salons and barbershops probably increases the risk of cancer and that long term employment in these establishments is "probably carcinogenic to humans." At the same time, the IARC, an arm of the World Health Organization, reported that there is not enough evidence to conclude that occasional personal use of hair coloring raises the risk of any type of cancer.
Beyond that, studies on the association of hair dyes with non-Hodgkin’s lymphoma and leukemia have shown no increased risks among people who color their hair using today’s dyes. Some increased risk has been observed among women who colored their hair before 1980 and used darker tints. Prior to that year, hair dyes commonly contained aromatic amines and other agents that caused cancer in animals. Those chemicals are not present in products on the market today.
In general, I discourage use of hair dyes containing artificial coloring agents, which to my mind are as suspect in cosmetic products as they are in food. More than 5,000 different chemicals - some of them carcinogenic in animals - are used in hair dyes; these can be absorbed through the scalp, with its rich blood supply and be carried throughout the body.
The major concern remains for salon workers who are exposed to hair dyes, both permanent and semi-permanent, on a daily basis, but even here the evidence of harm is still being debated and investigated.
Andrew Weil, M.D.