The results of small case control studies of organochlorine levels and breast cancer risk have been mixed. In a large European case control study (265 cases), a significant inverse relationship between levels of adipose DDE and risk of breast cancer was observed after controlling for known breast cancer risk factors; the authors did not evaluate PCBs.

In a case control study in Buffalo, New York, lipid-adjusted serum levels of DDE, HCB, Mirex, and total PCBs were evaluated among 154 incident breast cancer cases and 192 community controls. No evidence was seen of a positive association between levels of any of these compounds and breast cancer risk, with the possible exception of the less-chlorinated PCBs. Lopez-Carrillo et al. analyzed serum DDE levels in a case control study in Mexico, where the pesticide DDT is still in use. Serum DDE levels were not associated with risk of breast cancer. In one small study, however, contrary to expectation, the levels of octachlorinated dibenzo-p-dioxin (OCDD) were slightly elevated in the cancer cases, although no differences were observed for six other polychlorinated dibenzo-p-dioxin isomers.

Three prospective studies have used stored blood samples collected before diagnosis to evaluate the relationship between DDE and total PCBs, and breast cancer. In a cohort of 14,290 women in New York City (80% of whom were white), levels in sera from 58 women diagnosed with breast cancer within 1 to 6 months of blood collection were compared with levels among 171 controls. After adjusting for known risk factors, the odds ratio for women in the tenth decile of DDE levels compared with those in the first was 4.08 (95% confidence interval, 1.49 to 11.20), and the positive trend was statistically significant.

The equivalent odds ratio associated with PCB level was not statistically significant. In a prospective study of 57,040 San Francisco Bay area women who had provided blood in the late 1960s, when DDT and PCBs were still in production, 50 white women, 50 African-American women, and 50 Asian-American women with breast cancer occurring after blood draw and before 1991 were selected and compared with 150 control women matched for age and ethnicity. Risk of breast cancer was not associated with either DDE or PCB level when all ethnic groups were combined, although nonsignificant elevated risks were observed for DDE for African-Americans and whites. Among 236 women with breast cancer and their matched controls in the Nurses’ Health Study, no evidence was found of a positive association between breast cancer and either DDE or PCBs.

The multivariate relative risks for women in the highest quintile compared with women in the lowest were 0.72 (95% confidence interval, 0.4 to 1.4) for DDE and 0.66 (95% confidence interval, 0.32 to 1.37) for PCBs. For women in the highest quintiles of both DDE and PCB levels, the relative risk was 0.43 (95% confidence interval, 0.13 to 1.44) for joint exposure.

In summary, most large studies have not found evidence of increased breast cancer risk associated with blood levels of DDE or total PCBs; however, a small effect will always be difficult to exclude. All available studies address exposure to organochlorines in the decade or two before enrollment; it will be ery difficult to obtain data to address the hypothesis that childhood or even in utero exposure is associated with breast cancer risk 50 or more years afterward. Nonetheless, organochlorines appear unlikely to be major breast cancer risk factors or the explanation for rising breast cancer rates.

Electromagnetic Fields
Electromagnetic fields (EMFs) have been proposed to alter breast cancer risk, perhaps by altering melatonin secretion by the pineal gland. Although animal evidence is suggestive, few data address the relationship of melatonin levels to human breast cancer risk. Exposure to light at night has been shown to suppress melatonin secretion, and in some studies breast cancer risk has been lower among blindwomen.

Gathering high-quality epidemiologic data on EMF and nocturnal light exposure is challenging, and these questions are unlikely to be resolved definitively in the near future. Evidence of an elevated risk of male breast cancer associated with presumed occupational EMF exposure based on job title has been observed in some studies, but these results are based on small numbers of cases. No evidence of an increased risk of breast cancer was observed in the studies that also included female employees.

In case control studies designed specifically to study occupational exposure to EMF and breast cancer in women, small increases in risk have been inconsistently observed. However, in those studies, misclassification of exposure is a major concern. Because classifications are based on subjects’ “usual” occupation, often obtained from death certificates, duration of exposure and personal work tasks could not be accounted for in most of the studies, and adjustment for known breast cancer risk factors was limited or entirely absent.

The general population is exposed to EMFs primarily from power lines, transformer substations, and electrical appliance use. In an initial 1987 study of mortality from all cancer subtypes and residential wiring configurations, a statistically significant elevation in female breast cancer incidence was associated with magnitude of exposure at the current residence.

Other studies in Britain, the Netherlands, and Taiwan, however, did not observe an association between female breast cancer deaths and residence in the vicinity of electricity transmission facilities. Again, these studies are limited by the indirect methods used to assess EMF exposure.

Use of electric blankets (produced before 1990) throughout the night approximately doubles an individual’s average exposure to EMFs, because the blanket is placed close to the body. In one case control study, the use of electric blankets continuously throughout the night was associated with marginally significant increases for breast cancer in postmenopausal women (odds ratio, 1.46; 95% confidence interval, 0.96 to 2.20) and for premenopausal women (odds ratio, 1.43; 95% confidence interval, 0.94 to 2.17).

In a large case control study of breast cancer in women younger than age 55 years, however, no association was seen. Additional studies of electric blanket exposure and other residential exposures to EMFs and breast cancer risk are ongoing, but the biological plausibility and epidemiologic support for an important relation between EMF exposure and breast cancer risk are weak.