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Electromagnetic waves are a type of non-ionizing radiation, i.e., a type of low-frequency radiation without enough energy to break off electrons from their orbits around atoms and ionize (charge) the atoms. Microwaves, radio waves, radar and radiation produced by electrical transmission are examples of radiation sources that generate electromagnetic fields (EMF). Fluorescent lighting, computers and many other types of wired and wireless electronic equipment (e.g., cell phones) all create electromagnetic fields of varying strengths.

Both the IARC and the National Institute of Environmental Health Sciences (NIEHS) EMF Working Group have classified EMF exposures as possible human carcinogens based on the scientific literature related to EMF and childhood leukemias (NIEHS, 1998). More recently, data have suggested a link between EMF exposure, especially from cell phone use, and development of brain cancer and acoustic neuromas (Carpenter, 2010). However, consensus has been more difficult to reach about the relationship between EMF and breast cancer.

Research Exploring Links Between Non-ionizing Radiation and Breast Cancer Risk
Although many epidemiological or occupational studies have not found significant relationships between exposures to EMF and risk for breast cancer, others have reported data supporting these effects (e.g., McElroy, 2007; Peplonska, 2007). Methodological issues may account for some of the discrepancies, given the relatively small effects that are found and the ubiquitous nature of “background” EMF in our daily lives (Ahlbom, 2001).

One example of an occupational study that implicates EMF in increased risk for breast cancer is a study that reported an increased risk of breast cancer among female radio and telegraph operators exposed to radiofrequency (one type of EMF) and extremely low frequency EMF. Pre-menopausal women showed an increased risk of estrogen-receptor-positive tumors and post-menopausal women had an increased risk of estrogen-receptor-negative tumors (Kliukiene, 2003). Research has shown increased mortality from breast cancer in women employed in the telephone industry (Dosemeci, 1994), with pre-menopausal women being at higher risk than post-menopausal women (Coogan, 1996).

Studies of residential and occupational EMF exposure found a 60 percent increase in breast cancer risk among women of all ages living near high-voltage power lines. Occupational exposure also increased risk, but not as noticeably as residential exposure. Women younger than age 50 who were exposed to EMF both at home and at work had a modest increase in risk of breast cancer (Feychting, 1998; Kliukiene, 2004).

Nevertheless, two large meta-analyses have concluded that there is no clear relationship between EMF exposure and breast cancer in women (Chen, 2010; Erren, 2001).

Although breast cancer is rare in men, numerous studies point to a connection between EMF exposure and male breast cancer (Loomis, 1992; Matanoski, 1991; Milham, 2004; Tynes, 1992).

In the laboratory, EMF can cause increases in mammary tumors in animals and in vitro systems in which human breast cell tumors are grown in culture. Importantly, effects in rodents are found in some strains of animals but not others, indicating that subtle differences in genetic background might make some animals more susceptible to the carcinogenic effects of EMF (Fedrowitz, 2004). In an in vitro cell system, EMF exposure of human breast tumor (MCF-7) cells led to an activation of genes that have been associated with the induction of metastasis in breast cancer cells (Girgert, 2009). 
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