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At the same time, the study authors caution that this may not be the final word on the subject - more research may be needed on a tiny link seen between hair dyes and cancers such as leukaemia and multiple myeloma.
Study a summary of knowledge on the topic
"Our study is just the best summary of current scientific knowledge on the topic," said study author Dr Bahi Takkouche, a preventive medicine professor at the University of Santiago de Compostela. "One can never rule out that, in the future, several individual studies will find strong harmful effects. Still, the news is quite reassuring for hair dye users."
"The data should give reassurance to people that the risks of modern hair dye usage as a predisposition to cancer are, at most, very minimal," added Dr Robert Morgan Jr., section head of medical gynaecologic oncology at City of Hope Cancer Centre in Duarte, California.
Previous epidemiological studies had suggested that hair dye might be a risk factor for several different types of cancer, including bladder cancer. Researchers had speculated that compounds called aromatic amines, which are contained in the dyes, might be responsible for the association.
According to the authors of the current study, about one third of women in Europe and North America colour their hair, as do about 10 percent of men over the age of 40. The majority (about 70 percent) use permanent dyes.
Data from 79 studies was analysed
For this paper, which appears in the May 25 issue of the Journal of the American Medical Association, the researchers analysed data from 79 studies performed in 11 countries. Overall, they found no increased risk of breast and bladder cancer in people who used hair dye.
There was a small increased risk for blood cancers, but the association was a weak one.
"The borderline increase in the risk for haematopoietic cancers is not 'robust,' and may be easily explained by publication bias," Takkouche said. "Publication bias exists when there are studies that are not published, either because the editors reject them, or because the authors themselves fail to send them for publication. These studies are generally negative."
Some small risk increases noted
There were also small risk increases seen in brain and ovarian cancer, but this association is also suspect.
"The results concerning brain and ovarian cancers are based on minimal sample sizes (only two studies in each case), [so] they are not as reliable as the results of the rest of the cancers we studied," Takkouche said. "Besides, they are rare cancers, and the increase in the risk, even if it exists, would be of limited interest from the public health point of view."
"Epidemiologic studies are very difficult to perform and have previously implicated many potential sources of increased risks of these cancers without any definitive conclusions," Morgan added. "These studies are primarily hypothesis-generating and allow scientists to focus research on potential cancer-causing agents."
There was also a slightly increased risk of haematopoietic cancers in male users of hair dye. "This is an unexplained finding," Takkouche said. "We do not know whether the increase is real, or whether it is due to methodologic issues."
In addition to looking at male users of hair dye, Takkouche said that future research should be focused on the possible cancer-promoting effects of occupational exposure to hair dyes, such as that encountered by hairdressers and cosmetologists. Here, the exposure is higher and more prolonged than with personal use, Takkouche noted. – (HealthDayNews)
