Active and Passive Smoking
The relationship between active cigarette smoking and risk of breast cancer has been extensively evaluated in both case control and cohort studies; collectively, the data provide strong evidence against any major overall relationship. Initiation of smoking early in adolescence, when breast tissue may be maximally sensitive to carcinogenic influences, has been associated with an increased risk in a large case control study. However, in the largest case control study to date, no association with smoking at an early age was observed.

Passive smoking has been suggested to be an important risk for breast cancer, in part because sidestream smoke contains more carcinogenic activity per milligram than mainstream smoke. In a cohort study of cancer mortality among Japanese women exposed to passive smoke at home, a slight and insignificant risk elevation was seen (crude relative risk, 1.3; 95% confidence interval, 0.8 to 2.0). In several case control studies, increases in risk of breast cancer have been seen, but usually without evidence of a dose response.

Despite these positive associations, the absence of an effect of heavy smoking for decades is difficult to reconcile with an effect of exposure to much lower amounts of environmental smoke. A likely explanation for the positive association seen in case control studies is methodologic bias related to the selection of controls or the retrospective recall of exposure to passive smoke. However, data from large prospective studies is required to resolve the issue.

Silicone Breast Implants
Most studies examining the relation of silicone breast implants to breast cancer risk have actually reported lower rates of breast cancer among women with implants; thus, a direct association between silicone breast implants and the occurrence of breast cancer is unlikely.

Summary of Evidence on Environmental Pollution and Breast Cancer Risk
In general, evidence does not support any substantial relationship between exposure to humanmade chemicals or electrical fields in the environment and breast cancer risk. The most recent evidence in prospective analyses does not support an association between exposure to organochlorines and breast cancer risk.

Although occupational studies of EMF exposure have been inconclusive, residential studies imply that no risk is associated with overhead power lines. Measurable increases in breast cancer risk due to passive smoke seem implausible because of the lack of effect of active smoking, but prospective data are needed.

Although other environmental exposures that have not been identified may warrant evaluation, with the exception of ionizing radiation, no environmental exposure can be confidently labeled as a cause of breast cancer.

Walter C. Willett, Beverly Rockhill, Susan E. Hankinson, David J. Hunter and Graham A. Colditz

W. C. Willett: Harvard Medical School, Boston, Massachusetts; Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts
B. Rockhill: Harvard Medical School, Brigham and Women’s Hospital, Boston, Massachusetts
S. E. Hankinson: Departments of Medicine and Epidemiology, Harvard Medical School and Harvard School of Public Health, Boston Massachusetts
D. J. Hunter: Departments of Epidemiology and Nutrition, Harvard School of Public Health, Boston, Massachussetts
G. A. Colditz: Department of Medicine, Harvard Medical School, Boston, Massachussetts

References