The work suggests the increased risk is likely the result of exposures to chemicals used in some factory-type jobs and second-hand smoke in entertainment sector jobs, plus shift-work across a variety of these job sectors.
The research was led by James Brophy and Margaret Keith, both of whom have appointments at both the University of Windsor and the University of Stirling, in Scotland.
The study was the result of a multi-year research project funded by the Canadian Breast Cancer Foundation. It is published in the journal Environmental Health.
Brophy said the issue of workplace exposures influencing one's risk of developing cancer is under-appreciated in Canada.
"Nobody is paying attention to this," he said in an interview.
"And I think that's our take-away from this: We need to be much more serious about the kinds of exposures particularly blue collar women are getting. Because they're the most highly exposed population. And if there's an effect, they're a population that literally is acting as the canary in the coal mine."
Canadian regulations on exposure to chemicals such as bisphenol A "are completely out of date," Brophy said.
"They don't reflect what the current understanding of science is."
The researchers looked at women in Essex and Kent counties in southern Ontario, a part of the province where agriculture and heavy industry are found. As early as two decades ago, health officials in the area raised concerns about what appeared to be an excess rate of breast cancer in the region.
In this study, the scientists looked at the workplace exposures of over 2,000 women. Roughly half had been diagnosed with breast cancer. The other half were women who didn't have breast cancer but were similar to the other women in age and other factors.
By comparing the two groups, the researchers hoped to find clues to why the rates of breast cancer were higher in some industries than in others.
A study of this type cannot prove that something — exposure to a particular solvent, say — causes breast cancer. It can only suggest that the exposure is or isn't linked to a higher risk of cancer.
But in many cases this study points to chemicals or conditions for which there is other compelling evidence of an elevated cancer risk, for example experiments in animals.
The study found that for pre-menopausal women, the risk of developing breast cancer was five times higher for women who worked in the automotive plastics industry or in food canning than for women who didn't work in those sectors.
Furthermore, the exposures appeared to influence the type of breast cancer the women who got cancer developed. For instance, women who work in agricultural jobs appear to be at a high risk of developing estrogen negative, progesterone negative tumours, Brophy said.
Teasing out information that could link the types of breast cancers to the types of exposures may shed light on why the cancers develop, he said.
Overall, women in these types of blue collar jobs have double the risk of developing breast cancer than women who work elsewhere, the study suggested.
In the study, women who worked in bars and gambling facilities were twice as likely as other women to develop breast cancer. That's likely related to exposure to second-hand smoke and night-shift work, Brophy said. (Studies have suggested that shift work can effect the endocrine system, increasing cancer risks.)
The elevated risk in women in these jobs may lower over time, now that smoking has been banned in bars and casinos, Brophy admitted, though he suggested the shift-work component of those jobs may continue to influence breast cancer rates for those workers.