The possibility that occupational exposures, particularly to organic solvents, might explain the progressive increases in female breast cancer seen over the past 25 years (yr) was discussed. Breast cancer incidence rates in Canadian women have increased by 27.9% over the past 25yr, with even steeper increases occurring in women over the age of 50yr. The increase in female breast cancer in Canada has been paralleled by similar increases in other countries. It was noted that changes in the pathologic definition of breast cancer, earlier detection in mass screening programs operating in many western countries, and secular changes such as having fewer children and at later times can explain only part of the increase. A hypothesis was presented which proposed that increased occupational exposure to organic solvents due to the increasing number of women that have entered the workforce in the past few decades can be related to the increased incidence of breast cancer. Organic solvents and their metabolites, because they are lipophilic substances, can migrate to adipose tissue in the breast where they can be stored, further metabolized in-situ, and then excreted into the ductular systems where they may remain in contact with the breast parenchyma for prolonged periods of time, thereby initiating or promoting carcinogenesis through genotoxic or related mechanisms. Evidence supporting this hypothesis was discussed. This included the fact that the breast parenchyma is embedded in a fat depot capable of storing lipophilic xenobiotics and the results of studies showing that many organic solvents have been detected in breast milk, most carcinomas occur in the ductular system, and a range of organic solvents have been shown to cause mammary tumors in rodents.

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