A recent case-control study conducted in Canada has identified certain occupations that increase women’s risk of developing breast cancer, probably through exposure to endocrine disrupting chemicals and carcinogens at key stages in the life cycle.

Although it has long been known that lifetime exposure to endogenous and exogenous estrogen affects breast cancer risk, the link between occupation and endocrine disrupting chemicals and carcinogens is poorly understood. In particular, it is not known whether exposure during critical periods of breast development when breast tissue is less differentiated represents an especially high risk.

In the case-control study, researchers collected occupational and reproductive histories from 1,006 women with a history of breast cancer and 1,146 matched controls. All participants resided in two counties in southern Ontario with both agriculture and industrial manufacturing in which a clustering of excess breast cancer cases has persisted over time.

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The risk of breast cancer increased with the duration of work in agriculture or plastic manufacturing. A single year of work in the plastics manufacturing industry was estimated to increase the risk of breast cancer by 9%. Ten years of employment in agriculture, plastics, metal work, food canning, or bars/gambling substantially increased breast cancer risk.

The effect of these occupations on breast cancer risk was highest in women who worked in these fields during the time between their first full-term pregnancy and menopause. A slightly lower but still significant risk was seen for women who worked in metals, plastics, or bars/casinos during the time from menarche to first pregnancy.

In women with breast cancer, there were distinct associations between occupation and the estrogen receptor (ER) and progesterone receptor (PR) status of the tumor. Working in agriculture, metals, bars/gambling, or plastics was associated with ER+/PR+ breast tumors, and agriculture had an even stronger association with ER- tumors. For those who had worked in the food canning industry, the excess breast cancers appeared to be entirely ER+/PR- or ER-.

The odds of having a history of breast cancer were higher in women who had not finished high school, who were at lower income levels, and who had a smoking history. Previous studies have shown a higher risk of breast cancer in higher-income groups; in this population, lower income may have been associated with working in occupations with higher exposure to endocrine disrupting chemicals and carcinogens.

Although the study was not designed to identify likely carcinogens, the following are likely culprits: pesticides in agriculture; estrogenic chemicals and additives in the plastics industry; metallic fumes, metalworking fluids, and solvents in the metals industry; emissions from heated coatings in the canning industry; and second-hand tobacco smoke in bars and gambling.

A similar study in France produced somewhat different results. There, women in some white-collar occupations, chemical manufacturing, and the motor vehicle industry were at increased risk for breast cancer, whereas women who worked in agriculture were at lower risk.

Night shift work—which was not investigated in the Canadian study—also may increase the risk for breast cancer.

In the future, workplace exposure standards may need to consider not only the possible carcinogens that women are exposed to, but the life-stage during which they are exposed.

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