Researchers find few clear environmental links to breast cancer
A new report released on December 7 by the Institute of Medicine found little evidence of a clear-cut relationship between breast cancer and environmental factors. The institute is an independent group that is a component of the National Academy of Sciences, which advises the government and public. The exhaustive, 364 page report was prepared to address public fears regarding possible links between breast cancer and the environment finds; it found evidence strong enough to make only a few strong recommendations, most already well-known and none with a large proven benefit.
The most consistent data found by the investigators was that women can reduce their breast cancer risk by avoiding unnecessary medical radiation, avoiding combination hormone replacement therapy (HRT) with both estrogen and progesterone for menopause, limiting alcohol intake, and minimizing weight gain. (Avoiding obesity appears helpful only in preventing postmenopausal breast cancers, not those in younger women.) Overuse of computerized axial tomography (CAT or CT) scans, which deliver a relatively high dose of radiation, was a particular concern; however, the report noted that women should not avoid screening mammograms, which administer a much smaller dose.
The report, which took two years to prepare, was developed by a committee of 15 outside experts, primarily from universities, and nine institute staff members. The sole sponsor was a breast cancer advocacy group, Susan G. Komen for the Cure, which requested the report and spent $1 million on its preparation.
For women who were hopeful of definitive safety information about the massive number of chemicals to which individuals are exposed (i.e., air pollution, food contaminants, drinking water contaminants, cosmetics, and cleaning agents) the report may come as a disappointment. It is based largely on a review of existing research; thus, its limited advice reflects the lack of solid scientific information in many areas of concern to the public.
“In the last 20 years, the National Institutes of Health and private foundations have put a lot of money into trying to identify what are the risk factors for breast cancer,” said Irva Hertz-Picciotto, chairwoman of the expert committee and chief of environmental and occupational health at the University of California, Davis. “It’s a bit disappointing that so little has been learned with the amounts of money that have gone into it.” He noted that the committee could not “identify a bunch of environmental factors” that might contribute to breast cancer.
The problem is difficult to evaluate for a variety of reasons. Chemicals that are suspect of being toxins cannot ethically be given to people to see if they cause cancer. Individuals with past exposures can be studied; however, information regarding dosage and exposure duration may be vague. Animal studies can provide useful information; however, the results may not apply to humans. Furthermore, individuals are often exposed to mixtures of chemicals that may interact in complex ways, with effects that may also vary depending on a person’s genetic makeup.
Even for women who completely follow all the advice presented in the report, there are no guarantees. The study authors note: “The potential risk reductions from any of these actions for any individual woman will vary and may be modest.” The recommendations might entail tradeoffs. Moderate drinking may reduce the risk of heart disease; however, women who totally abstain lose that benefit, note the investigators. Another example, avoiding a recommended CAT scan might result in the failure to diagnose a serious health problem, which would be much more of a health hazard than the radiation from the procedure.
Additional recommendations in the report, based on relatively weaker evidence than those regarding radiation and HRT, include increasing exercise and avoiding smoking. The investigators also found “possible associations” between breast cancer and secondhand smoke, night shift work and chemical exposures (benzene, ethylene oxide and 1,3-butadiene, which are present in some workplaces), exhaust fumes, gasoline fumes, and tobacco smoke.
An even lower level of evidence was “biological plausibility,” which means that researchers can see a mechanism in animals whereby certain substances might cause breast cancer; however, insufficient evidence currently exists to establish the degree of risk for humans. One toxic chemical is bisphenol A, (BPA), which is used in some plastic containers, can liners, food packaging, and other products. The substance can mimic estrogen, which can feed the growth of some breast cancers. Dr. Hertz-Piccioto noted that women should avoid BPA as much as possible; however, they should carefully select substitutes because they might be more harmful than BPA.
The panel did not link hair dyes to breast cancer risk; however, they made no determination about possible risks to hairdressers, who may have regular, heavy exposure to the dyes. The authors noted that more information was also needed about nail salon workers and consumers exposed to various manicure products, which contain multiple hazardous chemicals.
The investigators noted that much additional research was indicated regarding the effects of various environmental exposures at different stages of life because the vulnerability of breast tissue may vary during childhood, adolescence, adulthood, before and after pregnancy, and even before birth. Dr. Hertz-Picciotto explained that research on women exposed to radiation during childhood reported an increased risk of breast cancer and suggested that young girls may be particularly susceptible.