Breast cancer rates have risen in the United States; between 1973 and 1998, rates increased by more than 40 percent. Identified breast cancer risks cannot account for this escalation. That means that additional, unknown causes must be at work. If we can find out why incidence is increasing, we can learn to prevent future disease.
Where should we look for clues to prevention? Synthetic chemicals that began pouring into the marketplace after World War II are one promising direction, because laboratory studies point to three mechanisms that could link various chemicals to breast cancer: chemical carcinogens can damage DNA; tumor promoters can make cells grow; and developmental toxicants can leave mammary glands more vulnerable to carcinogens.
After synthesizing data from national and international sources, Silent Spring Institute researchers have identified 216 chemicals that caused increased mammary tumors in an animal study. An estimated 100 of these chemicals are likely common sources of exposure in our everyday lives. For example, we are exposed to carcinogenic polycyclic aromatic hydrocarbons, or PAHs, in grilled and smoked food, tobacco smoke, and air pollution from auto exhaust, power plants, and other fossil-fuel-burning processes. Ethylene oxide is still commonly used in hospitals and other medical facilities to sterilize instruments. Other mammary carcinogens are found in certain furniture finishes, dyes, and solvents.
Once a cancer begins, other chemicals, called tumor promoters, may stimulate growth. We have known for years that natural estrogens and pharmaceutical estrogens—such as hormone replacement therapy—affect breast cancer risk. We now know that synthetic chemicals can also make human breast cancer cells proliferate in laboratory studies. Drs. Ana Soto and Carlos Sonnenschein at Tufts University pioneered the study of estrogen mimics in breast cancer cells after they realized that a chemical was leaching from the plastic tubing in their experiments and causing cell growth. Estrogen mimics are part of a larger group of chemicals known as endocrine disrupting compounds, or EDCs, because they affect hormones. EDCs are in such common commercial products as plastics, pesticides, detergents, and cosmetics.
Scientists at the U.S. Environmental Protection Agency discovered a third way that chemicals may increase breast cancer risk. When they dosed laboratory animals with the pesticide atrazine or other EDCs during certain weeks of pregnancy, the offspring never developed fully mature mammary glands, leaving the daughters more vulnerable throughout life to carcinogenesis.
Making the link from strong laboratory evidence to epidemiologic studies in women remains a challenge, partly because it’s so difficult to measure exposure to a complex mixture of pollutants over a lifetime. But laboratory studies can help us to target precautionary public health policies to reduce exposure and to identify urgent areas for breast cancer studies.
For Further Information
Published as an "Expert Perspective" in the April-June issue of Breast Diseases A Year Book Quarterly, Environmental Pollutants and Breast Cancer: The Evidence from Animal and Human Studies is a concise (3 pages) summary of our extensive review of the evidence from animal and human studies, published last year in Cancer.
The Environment and Breast Cancer: Science Reviews is a comprehensive report on what we know now about the links between environmental pollutants and breast cancer risk.