Technically, the term "electromagnetic field" (EMF) refers to all fields throughout the electromagnetic spectrum. In common usage, however, the term usually refers to so-called extremely low-frequency nonionizing radiation fields—those fields below 300 Hertz (Hz)—and often only to those fields in the 50 to 60 Hz range, which are also known as power-frequency EMFs. As a type of nonionizing radiation, EMFs in this range do not have sufficient energy to remove an electron from an atom or molecule, but generally transfer thermal energy to other particles. Power-frequency EMFs are those generated by electric power delivery systems—those for which there has been the greatest public concern and research about possible adverse human health effects.
Power-frequency EMFs have two components: electric fields and magnetic fields. The electric fields are generated from potential energy, or the presence of voltage on a power line. The magnetic fields, on the other hand, are generated from the actual electrical current, or the flow of electricity. Thus, when a standard household electric light is plugged into a live electrical socket, but turned off, it generates only an electric field. Once turned on, it generates both electric and magnetic fields, since the voltage is still present but current is now flowing. The size of a magnetic field increases as the amount of current flow increases, as the size of the source increases, and as one gets nearer to the source. Adverse health effects from acute exposures include shocks, burns, and death (by electrocution). Generally, from chronic exposures, only magnetic fields have shown associations with adverse human health effects in epidemiologic studies.
Until the late 1970s, it had been assumed that power frequency EMFs were too weak, or had too little energy, to cause biologic effects. Then, in 1979, Nancy Wertheimer and Ed Leeper published an epidemiologic study that showed that children in Denver, Colorado, who died of cancer, particularly leukemia, were more likely to live in houses with higher EMF exposures than children of similar ages living in the same neighborhoods. This set off a flurry of research seeking to determine whether those with high exposures to magnetic fields at home or in the workplace might be at greater risk of getting and/or dying of cancer than those with lower exposures. In general, the most consistent and compelling cancer data are found in the studies of children exposed to EMFs in their homes. While there is still much controversy, the most well-conducted studies suggest a slight excess of childhood leukemia, particularly among those most highly exposed. In workers, the data show weaker support for excess leukemia and brain cancers. A variety of other adverse effects have also been investigated, including adverse reproductive outcomes, neurodegenerative diseases, and cardiac abnormalities. However, there are fewer of these studies, the studies have more methodologic limitations, and the results are more uncertain.
There are some methods for reducing exposure to magnetic fields. For large sources, such as outdoor power lines, sometimes similar strength currents can be set up to run in opposite direction to each other so that they cancel out most of the magnetic fields. For smaller sources, such as appliances (e.g., electric blankets, hair dryers, televisions), the magnetic fields fall off rapidly with distance, so increasing the distance from a source rapidly reduces exposure.
National Research Council (1997). Possible Health Effects of Exposure to Residential Electric and Magnetic Fields. Washington, DC: National Academy Press.
NIEHS Working Group (1998). Assessment of Health Effects from Exposure to Power-Line Frequency Electric and Magnetic Fields, ed. C. J. Portier and M. S. Wolfe.
Reilly, J. P. (1998). Applied Bioelectricity. New York: Springer-Verlag.