Many risk factors for breast cancer are related to exposure to estrogen and other hormones that play a role in a woman's menstrual cycle. These risk factors include early menstruation, late menopause, having children late in life, never having children, and never breastfeeding. Pharmaceutical hormones and behaviors such as alcohol use and exercise that affect hormone levels also affect risk. Risk increases with age: About 77 percent of women with breast cancer are over 50 at the time of diagnosis. Genetic risk, family and personal history, and socioeconomic status are all associated with breast cancer risk. Ionizing radiation is also an established risk factor.

Silent Spring Institute researchers are examining other factors, such as exposure to synthetic estrogens and chemicals that cause mammary carcinogens in animals, that might underlie unexplained variation in breast cancer risk.

Do known breast cancer risk factors account for all breast cancer?

Less than half of breast cancer risk is explained by the known risk factors. The role of the breast cancer genes discovered thus far - BRCA1 and BRCA2 - is estimated at about 5-10 percent of cases. A recent study of 45,000 twin pairs in Scandinavia yielded estimates that genetics could ultimately explain about one-fourth of breast cancer risk. If one identical twin is diagnosed with breast cancer, the chances are 2 in 3 that the other will not have the disease.

What is estrogen's role in breast cancer risk?

Estrogen has an important role in breast cancer. Prolonged exposure to estrogen (from early menstruation or late menopause, for example) during a woman's lifetime is known to increase her susceptibility to breast cancer. Factors that appear to increase estrogen levels, including alcohol use, lack of physical exercise, higher body mass after menopause, and obesity, are associated with higher breast cancer risk.

What are endocrine disruptors (EDCs), and why are they potential risk factors?

EDCs are compounds that mimic or otherwise interfere with natural hormones. Although their names may be unfamiliar - for example, chemicals in the alkylphenol and phthalate families - they are in everyday products, including some pesticides, detergents, and plastics. EDCs that mimic estrogen are of particular concern because of the potential links to women's health.

Scientists have known for years that natural estrogen from the menstrual cycle is associated with higher breast cancer risk, and some newer studies show increased risk associated with recent or long-time use of certain pharmaceutical estrogens, including oral contraceptives and hormone replacement therapy. Other drugs have complex estrogen-related effects. For example, tamoxifen is used as a breast cancer therapy because it blocks estrogen in the breast, while at the same time, it increases cancer risk in the uterus.

So far, 100 synthetic compounds in industrial and commercial products have been identified as estrogenic, including many that have been specifically shown to make estrogen-dependent human breast cancer cells grow in the lab. While many of these chemicals are relatively weak estrogen mimics, exposure to complex mixtures of them is ubiquitous. As a result, we believe that estrogens in the environment are a priority for breast cancer research.