A study explores the occupations with the strongest links to breast cancer.
As the most common cancer among women, breast cancer’s potential triggers — from genes to behaviors to lifestyle and even environmental exposures — are relatively well studied. But researchers in Canada were interested in understanding how one factor in particular — the work environment — can affect a woman’s risk of developing the disease, and found that some jobs that expose workers to potential hormone-disrupting chemicals and carcinogens may contribute to a 42% higher risk of developing breast cancer compared with women who did not work in those settings.
The study included 1,006 women with breast cancer and 1,146 women without the disease living in Essex and Kent counties in Ontario, where there is a diverse distribution of agricultural and industrial occupations. The region is also known to have a relatively high cluster of persistent breast-cancer cases, which makes it a useful location for studying the effects of different occupations on risk of the disease.
Reporting in the journal Environmental Health, the researchers say that over a period of 10 years, women involved in manufacturing car plastics, canning food, as well as those who spent time in bars, racetracks or casinos, more than doubled their risk of breast cancer compared with those with other occupations. Women who worked in agriculture experienced 34% greater odds of developing breast cancer compared with those not in farming, which the authors believe reflects the longer time that women may be exposed to potentially cancer-causing compounds in those settings, since most farmers start working at an early age. Previous studies provide some hints as to why workers in these jobs may be particularly prone to higher cancer rates: exposure to pesticide residues during food-canning processes might be responsible for triggering breast tumors among workers, while secondhand smoke was the primary culprit in bars and gambling establishments. Potentially carcinogenic vapors emitted during the molding process of plastic were likely contributing to higher breast-cancer rates among workers in plastic manufacturing.
Even after the scientists adjusted for factors that are known to influence breast-cancer risk, such as smoking, weight, the number of children the women had, when they began menstruating, when they experienced menopause, whether they used hormone-replacement therapy and whether they relied on oral contraceptives, the strong association between certain occupations and breast-cancer risk remained. And the heightened cancer risk is likely connected to the jobs themselves, rather than to exposure to potential carcinogens in the region, since women living in the same area but with different occupations did not show as high a rate of the cancer.
The connection, however, does not imply that all women who work in farming or manufacturing will get breast cancer. These jobs tend to increase women’s exposure to known breast-cancer triggers, but other factors, such as genes, the length of exposure and method of exposure, may also be important.
One such factor may be shift work, which has been linked to increased vulnerability to cancer due to changes in cancer-contributing hormones. “A lot of factory work can be night-shift work, so what you’re seeing may be a relationship between night-shift work and cancer, not necessarily [a relationship to] a particular type of job,” says Dr. Wendy Y. Chen, a breast-cancer expert at the Dana-Farber Cancer Institute in Boston. The same could be true of women working in bars and gambling establishments. While the authors did investigate the potential role of night-shift work on cancer risk, they did not find statistically significant results connecting the two.
The small number of cases in each job category also means the results may not be robust enough for women to make any drastic changes in their occupations just yet, says Chen. While the study involved more than 1,000 participants with breast cancer, there were far fewer cases in each of the eight major job sectors in which the women worked, and even smaller numbers in the 32 more-specific job activities, such as “agriculture/plants,” “agriculture/animals” and “plastics manufacturing (auto).”
“Most of the studies we’d consider definitive about cancer in general would have much larger case numbers,” she says. “It looks like a big study because you see it’s got 1,006 breast-cancer cases and 1,146 controls, but it’s not such a big study once you break it up into all these little subgroups. [The study] is not something we would go ahead and make public-health recommendations on like, ‘If you work in farming, you should get more mammograms.’”
Even so, the findings suggest that doctors should be asking women about their occupations and pay special attention to those who work in potentially high-risk fields like farming and manufacturing.
“Not only should doctors ask what work women do or did recently, but also they should take a complete work history so they know what occupations and what industries or employment sectors women worked in throughout their lives,” co-author Andrew Watterson, chair of health effectiveness at the University of Stirling, in the U.K., wrote in an e-mail response discussing the findings. “Doctors may ask what work women did but don’t collect the complete work history, which our study shows can be influential in increasing or reducing breast-cancer risk.” By adding occupational history to family history and other health data that doctors now gather from patients, they may eventually form a clearer picture of what does and does not contribute to breast-cancer risk.