Ref. #15102

Breast Cancer and the Environment

An Overview of Current Medical and Scientific Understanding

According to the American Cancer Society (ACS), an estimated 184,300 new invasive cases of breast cancer will be diagnosed among women in the U.S. in 1996. ACS reports that breast cancer incidence rates for women increased about four percent a year between 1982 and 1987, but recently have leveled off at about 110 cases of breast cancer per 100,000 women. National Cancer Institute (NCI) data collected through 1994 also report that breast cancer incidences have leveled off. NCI estimates that among women who reach the age of 85, one in nine will develop breast cancer.

Both ACS and NCI acknowledge that most of the recent increase in rates is believed to be due to marked increases in mammography utilization. However, the reasons for longer-term trends in breast cancer are not fully understood.

Recent NCI research on breast cancer incidence suggests that "a substantial proportion of breast cancer cases in the U.S. are explained by well-established risk factors (i.e., later age at first birth, never giving birth, family history of breast cancer and higher socioeconomic status)." In an NCI study evaluating geographic factors in the risk of breast cancer, it was concluded that most of the geographic variation in breast cancer mortality appears due to known reproductive and other risk factors.

NCI researchers acknowledge, however, that existing data only explain about half of the incidences of breast cancer, concluding that, "if the genetic, hormonal and other biological exposures and traits underlying breast cancer risk factors can be understood, these determinants may be responsible for a considerable part of the current unexplained fraction of breast cancer cases as well."

In the search for ways to prevent breast cancer, some scientists and advocates have raised questions about possible environmental links to breast cancer. These questions are important and deserve - and are getting - thorough investigation. A recent hypothesis focuses on how some naturally occurring and synthetic chemicals in the environment might mimic the effects of one of the body's own natural hormones, estrogen - either alone or with other hormones - to promote the growth of breast cancer cells.

Certain common chemicals - including many other natural and man-made substances - may exhibit estrogen-like (estrogenic) properties. It has been suggested that exposure to estrogenic chemicals in the environment may add to lifetime exposure to estrogen which may thereby increase the risk of breast cancer. Scientists have raised the question - a hypothesis - of a possible relationship between specific chlorine-based chemicals (often called organochlorines) and breast cancer.

At the heart of the chlorinated chemical/breast cancer hypothesis is the suggested link between estrogen and the development of breast cancer. Hormones produced by the ovary, especially estrogen, are important factors related to breast cancer risk because they can cause breast cancer cells to proliferate. This indicates that the cancer is sensitive to hormones, but it does not mean that the cancer is directly caused by the hormone.

As summarized below, the current weight of scientific evidence does not support the hypothesis of a relationship between chlorinated chemicals and breast cancer. In a 1993 study, Dr. Mary S. Wolff first raised the chlorinated chemical/breast cancer hypothesis. Dr. Wolff's study examined two chlorinated compounds. PCBs (polychlorinated biphenyls) and DDE (dichlorodiphenyl dichloroethylene, a breakdown product of the now-banned pesticide DDT or dichlorodiphenyl trichloroethane). By comparing blood levels of these chemicals in women who did not have breast cancer with those of women with breast cancer, Dr. Wolff concluded that there may be a link with DDE. However, she concluded there is no link between PCBs and breast cancer.

The largest chlorine-related study conducted thus far - specifically designed to test Dr. Wolff's hypo-thesis - did not support a relationship between exposure to DDE or PCBs and increased risk of breast cancer. Led by Dr. Nancy Krieger of the Kaiser Foundation Research Institute, this study included Dr. Wolff as a key researcher.

The role of synthetic estrogens (those not produced naturally by the body) in breast cancer is uncertain. Dozens of studies of women using oral contraceptives (which contain synthetic estrogen) or undergoing estrogen replacement therapy at menopause have found no firmly established link between significant doses of therapeutic estrogens taken daily for years and increased breast cancer. In fact, a 1996 analysis of nearly all studies on the subject of the pill and breast cancer has concluded that the pill does not appear to increase the risk of breast cancer in women 10 to 20 years after they stop using it.

Scientific evidence on oral contraceptives and estrogen replacement therapy provides important clues in evaluating possible risks to people from environmental exposures to estrogen. Under medical prescription, women ingest significantly higher doses of estrogen than those which occur from dietary or environmental exposure. DDT, for example, is only weakly estrogenic - about one thousand to one million times less estrogenic than medically prescribed estrogen replacement therapy.

Studies by NCI, the International Agency for Research on Cancer, and other medical and scientific authorities have not been able to make a credible connection between normal "everyday" environmental exposures and increased breast cancer risks.

A 1995 Harvard School of Public Health review, supported by a grant from the Chlorine Chemistry Council (a business council of the Chemical Manufacturers Association), concluded that "available data do not indicate that organochlorines will affect risk of [breast cancer or endometrial cancer]." The review noted that it is questionable whether background levels of organochlorines in the environment are high enough to cause adverse effects. However, the reviewers - including researchers from Harvard, Dartmouth College and two Swedish scientific institutions - agreed that the hypothesis could not be rejected and suggested further research.

More recently, a 1996 report by the National Academy of Sciences, although not limited to breast cancer, concluded that (1) the risk of cancer from naturally occurring chemicals is much greater than the risk from synthetic chemicals in the environment and (2) for the majority of natural and synthetic chemicals found in food, the levels are far too low to pose a measurable cancer risk.

While the weight of scientific evidence does not support the hypothesis of a relationship between chlorinated chemicals and breast cancer, the Chlorine Chemistry Council (CCC) and its member companies encourage and are supporting additional research.

Ongoing CCC-supported studies include:

• The Medical College of South Carolina is nearing completion of a study investigating the effects of DDT on women exposed to high levels of DDT over many years. Study findings are expected to be published in the near future.

• Dartmouth College and Uppsala University (Sweden) are investigating a possible association between certain organochlorines and endometrial cancer in women. The study is one part of a larger NCI-sponsored investigation on women's health. Published results are expected in 1998.

The CCC is committed to furthering the public dialogue on breast cancer. As part of that commitment, the Council will continue to explore opportunities to support research initiatives that help us understand and find ways to prevent breast cancer.

For further information, contact:
Janet Flynn
Director of Public Affairs

Chlorine Chemistry Council
1300 Wilson Boulevard
Arlington, VA 22209
703/741-5827
703/741-6827 (fax)
http://c3.org