The latest word on this continuing concern comes from the International Agency for Research on Cancer (IARC), which has concluded that working regularly with hair dyes in hair salons and barbershops probably increases the risk of cancer and that long term employment in these establishments is "probably carcinogenic to humans."
However, the IARC, a World Health Organization panel, reported that there is not enough evidence to conclude that occasional personal use of hair coloring raises the risk of cancer.
This new report, published in the April, 2008, issue of Lancet Oncology, is the first scientific word on the subject since 2005, when Spanish researchers reported that their review of 79 studies from 11 countries yielded no strong evidence of a link between hair dye and cancer risk. The only connection the Spanish team observed was the possibility of a slight increase in the risks of leukemia and multiple myeloma, but they concluded that the causal effect was too weak to be a major concern.
Earlier, researchers at Yale University found that long-term use of dark hair dye by women who began coloring their hair before 1980 may increase the risk of non-Hodgkin's lymphoma. No such risk was seen among women who began dyeing their hair after 1980. Here, the difference may be due to the elimination of coal-tar-derived ingredients used in the older products that are known carcinogens. The Yale study was published in the Jan. 15, 2004, issue of the American Journal of Epidemiology.
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. Hair dyes applied to the head are absorbed through the scalp, where there's a very rich blood supply that may carry them throughout the body. I'm sure that the new IARC report won't be the last word on this subject. I'll keep you posted on developments.
Andrew Weil, M.D.