Breast Cancer and the Environment

Why study the environment in the search for ways to prevent breast cancer?
Until recently, most breast cancer research has focused on detection and treatment, with little attention paid to identifying preventable causes. Silent Spring Institute is looking for connections between breast cancer and exposure to harmful chemicals in the environment. Several lines of evidence show that we can identify ways to reduce breast cancer risk by finding additional environmental factors.

Inherited genes are not the predominant cause. Only 5-10% of breast cancers are associated with the high-risk BRCA1 and BRCA2 genes.

Risk varies geographically and increases when countries industrialize. If a woman born in a low-risk area, such as Japan, moves to a high-risk area, such as California, her breast cancer risk rises, and the risk increases again for her daughters and yet again for her granddaughters. Studies of unexplained geographic variation in breast cancer rates provide evidence that additional, modifiable risk factors can be identified.

In addition, laboratory science provides strong hypotheses about biological 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.

What is the estrogen link in breast cancer risk?
Increased exposure to natural estrogen over a woman’s lifetime—from early menarche, from late menopause, from having no children, from having children late in life, or from never breastfeeding—increases her risk of breast cancer. Pharmaceutical estrogens—such as hormone replacement therapy—also affect breast cancer risk. We know that synthetic chemicals can make human breast cancer cells proliferate in laboratory studies, and scientists are now asking whether exposure to chemicals that can mimic estrogen might also be linked to breast cancer. 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.

How do endocrine disrupting compounds affect health?
Endocrine disrupting compounds can mimic or disrupt our bodies’ hormone systems, thereby interfering with cell growth and development. Endocrine disrupting compounds are being studied to determine how they affect child development, reproduction, and hormonal cancers such as breast and prostate cancer.

What kinds of chemicals have been implicated in breast cancer?
The Silent Spring Institute Mammary Carcinogens Database includes 216 chemicals that increased mammary gland tumors in at least one animal study. Many are common 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. Until recently, ethylene oxide was commonly used in hospitals and other medical facilities to sterilize instruments, though the Occupational Safety and Health Administration has since set limits to reduce exposures. Other mammary carcinogens are found in certain furniture finishes, dyes, solvents, medications, and chlorinated drinking water.

Do other cancers and illnesses have environmental causes?
Yes. A number of diseases have documented environmental causes; these links include lung cancer and cigarette smoking, oral cancer and the use of chewing tobacco, mesothelioma and asbestos inhalation, leukemia and exposure to benzene, and melanoma and breast cancer and exposure to radiation.

What do wildlife studies tell us about the effects of the environment on development and reproductive health?
For several reasons, including the intimate relationship they have with the environment that surrounds them, wildlife are often the first to exhibit the harmful effects of environmental pollutants. They are sentinels, warning of dangers as yet undetected in other populations.

Many studies have uncovered adverse effects that pesticides and other hormone disruptors have on embryonic development or reproductive functioning in wildlife. Studies of male fish in rivers in Europe have shown, for example, that the fish developed both male and female reproductive organs after exposure to endocrine disrupting pollutants in their environment. Other, more recent research on the effects of atrazine have found that at low levels, the pesticide demasculinized tadpoles, causing them to develop as hermaphrodites—that is, to have both male and female sex organs and sexual characteristics. Atrazine also affects developing mammary gland in rats.