No Strong Link Between Breast Cancer Risk and Pollutants

BETHESDA, Maryland—Results from the largest epidemiologic investigation of possible links between two major types of environmental pollutants and breast cancer indicate a 50% increase in risk of the disease for women exposed to polycyclic aromatic hydrocarbons (PAHs) at the highest level. However, researchers failed to find an association between organochlorine compounds, which include DDT, and an increased risk of breast cancer.

The new findings emerged from two studies conducted as part of the Long Island Breast Cancer Study Project, which Congress mandated in 1993. The two studies involved a total of 1,058 women living in Nassau and Suffolk counties on Long Island, which have disproportionately high breast cancer rates, compared with other counties in New York State.

The goal of the two population-based, case-control studies "was to determine whether breast cancer incidence in these two counties was associated with exposures to environmental contaminants," said Marilie D. Gammon, PhD, who served as principal investigator for both studies. "What we observed did not support that possibility strongly." Dr. Gammon is associate professor of epidemiology, University of North Carolina School of Public Health, Chapel Hill.

PAHs result from incomplete combustion. They are inhaled with such things as cigarette smoke and from the smoke and vehicle exhaust that result from burning fossil fuels, and are ingested by eating grilled and smoked foods.

The compounds are potent mammary carcinogens in rodents, and the federal government has designated several of them as probable or possible human carcinogens. However, researchers have not clearly demonstrated carcinogenic effects in the breasts of women.

Organochlorines include several pesticides (such as DDT); the family of industrial compounds known as PCBs; and the termiticide chlordane. DDT was used widely on Long Island to control mosquitoes and gypsy moths before it was banned in 1972. Several previous epidemiologic studies have indicated a link between organochlorine exposure and breast cancer. However, most such studies have not supported an association of DDT and PCBs with the disease.

In the PAH study, Dr. Gammon and her colleagues focused on PAH-DNA adducts—places where the hydrocarbon attaches to the DNA in cells. Adducts serve as markers for exposure to a chemical and are widely believed to indicate tissue damage. The researchers looked for adducts in cells from blood samples donated by 576 women newly diagnosed with breast cancer and 427 women without breast cancer who served as controls.

"The age-adjusted odds ratio for breast cancer in relation to the highest quintile of adduct levels compared to the lowest was 1.51, with little or no evidence of confounding," the researchers reported. However, they found no consistent evidence of an increased risk of breast cancer with increased adduct levels, and no consistent association of adduct levels and two major causes of PAHs—active smoking and inhaling second-hand cigarette smoke, and eating grilled and smoked foods. "The data indicate that PAH-DNA adduct formation may influence breast cancer development, although the association does not appear to be dose dependent and may have a threshold effect," the team concluded.

Said Dr. Gammon, "Our findings suggest that women’s individual responses to similar PAH exposures might be more relevant to breast cancer development than the absolute amount of PAH," and the team is investigating this issue.

Organochlorines Study

For the study of organochlorines, the researchers analyzed blood from 646 women with newly diagnosed breast cancer and from 429 controls for exposure levels to the target chemicals. No substantial elevation in breast cancer risk was found for the lipid-adjusted serum levels between the highest and lowest quintiles for DDT, DDE, chlordane, and dieldrin and other PCBs. No dose-response relations were identified.

A woman’s risk was not increased in relation to organochlorines among those who had not breast-fed, or were overweight, postmenopausal, or long-time Long Island residents. Whether a woman’s breast cancer was diagnosed as invasive or in situ, or whether the tumor was hormone-receptor positive or negative failed to show an association with organochlorine exposure.

The researchers concluded that the results did not support the notion that exposure to organochlorines increases a woman’s risk of breast cancer. However, the study did not exclude the possibility that organochlorines play an early, subtle role in tumor development.

Dr. Gammon noted that some research suggests that the compounds may be related to breast cancers with clinical characteristics associated with poor survival. "We are continuing to investigate this issue among the women in our study," she said. The two reports appeared in the August issue of Cancer Epidemiology, Biomarkers & Prevention.