Some activist groups have hypothesized that exposure to chemicals in the environment may cause cancer in humans. While scientific research has never established such a link, a new study targets pesticide and other chemicals as possible explanations for high rates of breast cancer in some parts of Newton, Mass.

Researchers surveyed 1,350 Newton, Mass. residents living in areas of high and low breast cancer incidence. Similar to previous studies, researchers found that women who lived in the high incidence area generally had higher incomes, education levels and other indicators of socioeconimic status (SES). But, the Silent Spring Institute, which conducted the study, wanted to determine whether higher SES residents shared underlying environmental factors — namely pesticide and chemical use — that put them at higher breast cancer risk.

The Silent Spring Institute is named in recognition of Rachel Carson, author of Silent Spring, whose much criticized work tied the use of pesticides with adverse effects on wildlife. The Institute’s Web site indicates that their mission is to exclusively target the environment as the cause of women’s diseases.

“Obviously education and money do not themselves cause breast cancer,” remarked Dr. Nancy Maxwell, principle investigator of the study. “With the Newton Study we tried to learn about what may lie behind socioeconomic status.”

The researchers found that a greater percentage of women living in the high-incidence area reported using either a professional lawn service, termite treatments, and/or pesticides. Use of dry cleaning services was also speculated as a possible risk factor contributing to differences in breast cancer rates between high and low incidence areas. The Silent Spring researchers point to the “endocrine disrupting” compounds found in some pesticides as the possible culprit. Earlier this year, the National Academy of Sciences failed to find evidence to support this theory.

Scientists have identified factors, such as later age at first childbirth, and higher rates of mammography, that likely account for the differences in breast cancer risk between women of lower and higher SES. Greater lifetime exposure to the natural female hormone estrogen is associated with an increased risk for breast cancer. Higher SES women are more likely to have children at later ages, thereby increasing their lifetime exposure to estrogen and their risk for breast cancer. Higher rates of mammography are associated with increases in breast cancer incidence because screening detects breast cancer — often at early, more treatable stages.

According to the study, individual characteristics such as reproductive history and family history of breast cancer only accounted for 5 percent of the differences in high and low incidence areas. The researchers did note, however, that higher rates of mammography and breast self-exam reported in the high-incidence area likely contributed to the corresponding higher rates of breast cancer.

Katty Brody, Ph.D., spokesperson for Silent Spring, told ACSH that more research is needed to examine the relationship between environmental exposures to chemicals and breast cancer.

“Right now,” Dr. Brody stated, “we don’t know whether these factors contribute to breast cancer or not. We’re at a frustrating point in our knowledge about the causes of breast cancer.”

Some scientists are concerned that women may become unduly alarmed about the questionable findings of this study, which has not appeared in a peer-reviewed journal.

Toxicologist Dr. Daland Juberg cited lack of evidence supporting this association: “While there are established risk factors for breast cancer, including higher education and socioeconomic status, exposure to pesticides is not among them. Most pesticides that have been studies toxicologically, are very weakly estrogenic in nature, hundreds, if not thousands of times less potent than the body’s own natural estrogen. Epidemiological studies (those involving humans) do not support a causal relationship between pesticides and breast cancer.”

Juberg added, “This study proposes various hypotheses related to a better understanding of breast cancer incidence, but it did not measure human exposure to specific chemicals or biological agents. This would be among the first steps in a well-planned and scientifically defensible study aimed at elucidating the role, if any, of environmental agents in the cause of breast cancer.”

Juberg urged women to focus on well-established preventive screening strategies, such as breast self-exams, routine mammography, and regular medical checkups.