WEDNESDAY, Dec. 7 (HealthDay News) — Women may be able to reduce their risk of breast cancer by avoiding exposure to certain environmental factors, according to an Institute of Medicine report published Dec. 7.

Irva Hertz-Picciotto, PhD, a professor in the department of public health sciences and chief in the division of environmental and occupational health at the University of California in Davis, and colleagues reviewed current evidence on the relationship between environmental factors and breast cancer, the challenges in investigating these factors and potential preventive actions to reduce breast cancer risk.

Combined estrogen and progestin hormone therapy, exposure to ionizing radiation, excess weight in postmenopausal women and alcohol consumption were among factors researchers found were consistently linked with increased breast cancer risk. Some evidence suggests that physical activity may decrease the risk, they noted.

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Studies examining the connection between smoking and breast cancer had mixed results, as well as possible associations for other factors, including secondhand smoke and nighttime shift work. For certain environmental agents, such as bisphenol A, a clear mechanism has been identified in animals, but studies in humans were lacking or inadequate.

Research challenges included lack of understanding about breast development and the origins and progression of breast cancer, early lifetime exposures, the ethical challenges posed by randomized controlled clinical trials and the limitations of animal studies.

Preventive actions to reduce breast cancer risk include avoiding unnecessary medical radiation, postmenopausal combined hormone therapy and smoking; limiting alcohol consumption; increasing physical activity; and minimizing weight gain, particularly for postmenopausal women.

“The potential risk reductions from any of these actions for any individual woman will vary and may be modest,” the researchers wrote.

Institute of Medicine. “Breast Cancer and the Environment: A Life Course Approach.” Consensus Report. Dec. 7, 2011.