The medical records and hair-care patterns of 46,709 women between the ages of 35 and 74 were analyzed over an eight-year period, and the results were published in the International Journal of Cancer on last week.
However, cancer expert Shawn Chirrey, senior manager of analysis at the Canadian Cancer Society (CCS), worries the results could be slightly misleading.
“There’s no conclusive research saying that having your hair dyed will cause cancer,” Chirrey told Global News.
Currently, the International Agency for Research on Cancer (IARC) only recognizes a possible causal link between hair dye and bladder cancer for those in an occupational setting — working regularly as a hairstylist or barber.
The CCS lists hair-dressing as a possible risk factor for bladder cancer and non-Hodgkins lymphoma. The organization defines a possible risk factor as having been “linked” with a type of cancer, but without “enough evidence to show they are known risk factors.”
That’s because many hair dyes contain the chemicals trichloroethylene and benzene, which are known carcinogens.
As for personal hair dye use, IARC considers it to be “not classifiable as to its carcinogenicity to humans” due to a lack of evidence from studies in people.
Chirrey says the methodology used in this study could pose some problems.
“This was a large-scale epidemiological study, so it provides interesting data, but epidemiological studies of this nature, especially ones that are observational, are often weak for causal data,” said Chirrey.
There are two main types of studies used to determine if a substance causes cancer, according to the American Cancer Society. The first kind, done in a lab, exposes animals to a substance in large doses to see if it causes tumors.
The other type analyzes cancer rates in different groups of people. It’s common for experts to look at data from both kinds of studies for one substance, since neither type of study “provides enough evidence on its own.”
This is a study of the second variation. Trevor Dummer, associate professor in the faculty of medicine at the University of British Columbia, said it’s near-impossible to conduct a study of the first variation on humans because researchers can’t knowingly administer products that are potentially carcinogenic — the results could be life-threatening.
“It’s very difficult to associate a causal relationship from this type of study … because you can’t do that type of study design for something we suspect might harm you.”
Dummer thinks the study offers “some useful data,” but he is concerned about the population of women analyzed by researchers.
“All the women who were recruited … had a sister with a diagnosis of breast cancer. They were all themselves already at a higher risk,” he said.
This doesn’t mean the data isn’t helpful, he said, but it can make generalizing the data to the larger population more difficult.
“They’ve established behaviour patterns over time and then they recorded whether someone developed breast cancer,” he said. “They’re assessing whether the exposure … occurred before the cancer. That’s an appropriate design.”
There’s also some concern around asking subjects in a study to self-report their behaviour.
Ultimately, there needs to be more research about the long-term effects of hair dye use, in both personal and professional settings.
This one study shouldn’t lead people to avoid hair dye altogether, said Dummer, but it can improve awareness about what is proven to cause cancer and which risk factors are within a person’s control.
“We know obesity causes breast cancer. We know you should reduce alcohol intake to reduce your breast cancer risk.”
Dummer worries that media coverage of single studies like this one can overshadow what is actually known about cancer risk factors.
“One of the messages I think that people need to understand is we can all help to prevent cancer … cancer is a preventable disease sometimes.”