A case-control study draws some surprising conclusions about the deadly disease.

Basically all cancers are caused by a mix of genetic and environmental factors – with some cancers driven more strongly by one or the other. When it comes to ovarian cancer which kills more than 13,000 women per year in the US, genetic factors like the BRCA gene mutations are well-described.

Other risk factors, like early menarche and nulliparity, are difficult to modify. The only slam-dunk environmental toxin to be linked to ovarian cancer is asbestos. Still, the vast majority of women who develop ovarian cancer do not have a known high-risk gene nor asbestos exposure – so there may be other triggers out there. How do we find them? The answer may just be good old-fashioned epidemiology.

When you’re looking for a new culprit agent that causes a relatively rare disease, the case-control study design is your best friend.

That’s just what researchers, led by Anita Koushik at the University of Montreal did in a new study appearing in the journal Environmental and Occupational Medicine.

They identified 497 women in Montreal who had recently been diagnosed with ovarian cancer.

They then matched those women to 897 women without ovarian cancer based on age and address. A brief aside here – this approach would not work well in the US, as diagnosis of ovarian cancer might depend on access to medical care which is not universal here.  In Canada, though, its safer to assume that anyone who could have gotten ovarian cancer in Montreal would have been detected.

Cases and controls identified, the researchers took a detailed occupational history for each participant – every job they ever worked, and when, and for how long. Each occupation was mapped to a standardized set of industries and – interestingly – to a set of environmental exposures ranging from cosmetic talc to cooking fumes to cotton dust in what is known as a job-exposure matrix. They of course also collected data on other ovarian cancer risk factors.

After that, it’s a simple matter of looking at the rate of ovarian cancer by occupation and occupation-associated exposures, accounting for differences in things like pregnancy rates.

A brief aside here. I was at dinner with my wife the other night and telling her about this study and said “ok so what do you think the occupation with the highest rate of ovarian cancer is?” and without missing a beat she said “hairdressers”. Which blew my mind because of how random that was, but she was also (as usual) 100% correct.

Hairdressers, at least those who had been in the industry for more than 10 years, had a 3-fold higher risk of ovarian cancer than matched controls who had never been hairdressers.

Of course, my wife is a cancer surgeon, so she has a bit of a leg up on me here. Many of you may also know that there is actually a decent body of literature showing higher rates of various cancers among hairdressers, presumably due to the variety of chemicals they are exposed to on a continuous basis.

The number two highest risk profession on the list?  Accountants, with about a two-fold higher risk. That one is more of a puzzler. It could be a false positive – after all there were multiple occupations checked and random error might give a few hits that are meaningless. But there are certainly some occupational factors unique to accountants that might bear further investigation – maybe exposure to volatile organic compounds from office printers, or just a particularly sedentary office environment.

In terms of specific exposures, there were high risks seen with mononuclear aromatic hydrocarbons, bleaches, ethanol, and fluorocarbons among others – but we have to be a bit more careful here. These exposures were not directly measured. Rather, based on the job category a woman described, the exposures were imputed based on the job-exposure matrix. As such, the correlations between the job and the particular exposure are really quite high, making it essentially impossible to tease out whether it is, for example, being a hairdresser, or being exposed to fluorocarbons as a hairdresser, or being exposed to something else as a hairdresser, that is the problem.

But this is how these types of studies work – they tend to raise more questions than they answer. But in a world where a cancer diagnosis can seem to come completely out of the blue – they provide the starting point that, someday may lead to a more definitive culprit agent or group of agents. Until then, it might be wise for hairdressers to make sure their workplace is well-ventilated.

A version of this commentary first appeared on Medscape.com