Risk Factors for Breast Cancer

What are the recognized risk factors for breast cancer?
Ionizing radiation—including exposure from x-rays and CAT scans—is an established environmental risk factor for breast cancer.

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 menarche, late menopause, having children late in life, never having children, and never breastfeeding.

Pharmaceutical hormones, such as HRT and DES, and behaviors that affect hormone levels—such as alcohol use and exercise—also affect risk. Limiting alcohol use and increasing physical activity reduce risk. Women are at much higher risk than men, and risk increases with age. Inherited genes, family history, and socioeconomic status are all associated with breast cancer risk.

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

Do known risk factors account for all cases of breast cancer?
No. The role of the high risk breast cancer genes discovered thus far—BRCA1 and BRCA2—account for an estimated 5 to 10 percent of cases. A 2000 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 two in three that the other will not have the disease. One complicating issue is that risk factors don’t act alone; they may interact. That means it doesn’t make sense to say one or the other risk factor causes breast cancer. The culprit, for example, may be genes and the environment, rather than genes or the environment.

What role does estrogen play in breast cancer risk?
Estrogen and other hormones play an important role in breast cancer. Prolonged exposure to estrogen during a woman’s lifetime, such as from early menstruation or late menopause, is known to increase her susceptibility to breast cancer. Factors that appear to increase estrogen levels—including alcohol use, lack of physical exercise, a higher body mass after menopause, and obesity—are associated with higher breast cancer risk.

What are endocrine disruptors, and why are they potential risk factors?
Endocrine disruptors, or 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 their potential links to women’s health.

Scientists have known for years that natural estrogen from the menstrual cycle is associated with a higher breast cancer risk, and newer studies show an increased risk associated with recent or long-term 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 the risk of uterine cancer.

Already 900 synthetic compounds in industrial and commercial products have been identified as EDCs, including many that have been specifically shown to make estrogen-dependent human breast cancer cells grow in the laboratory. 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 should be a priority for breast cancer research.