the range of magnetic fields encountered is usually quite small, the fields are generally described in units of microtesla (1 µT = 0.000001 T) or milligauss (1 mG = 0.001 G). For example, the earth's geomagnetic field is a static field of about 50 µT (0.5 G), and a current of 50 amperes (A) in a straight wire produces a magnetic flux density (magnetic field) of 100 µT at a distance of 10 centimeters (cm). Although household alternating current in the United States has a frequency of 60 Hz, other relatively low-frequency electric and magnetic fields can be induced when the current is used to operate appliances, such as electric razors, hair dryers, video-display terminals, and dimmer switches.

Electric fields from direct exposure to high-voltage power lines and electric appliances induce current on or just within the surface of an exposed person's body. Because the electric fields are perturbed by the tissue conductivity, the fields inside the body are very weak. On the other hand, magnetic fields pass through the body and can induce electric currents throughout the body. Magnetic fields can pass through most common building materials, including thin sheets of metal. However, magnetic materials, such as iron and some metallic alloys, can serve as convenient paths for the conduction of magnetic fields and can be used as magnetic shields in some cases. People can be shielded quite easily from exposure to electric fields, because most materials possess sufficient conductivity to attenuate the fields.

Although electric and magnetic fields are quite different in character, time-varying fields are generally described together as electromagnetic fields. As noted above, time-varying electric and magnetic fields are formally linked and described mathematically by Maxwell's equations. Through coupling, a time-varying magnetic field induces an electric field and vice versa. However, in the limit of unchanging (static) fields, the electric and magnetic fields are independent. At the low frequencies associated with electric-power use, the coupling is extremely weak, and electric fields and magnetic fields can be considered independent to an excellent approximation. In this report, the term electromagnetic field (EMF) is used when the electric and magnetic fields are substantially linked, usually only for high-frequency fields.

Biologic Effects

Very-low-frequency electric and magnetic fields are known or suspected to interact with biologic systems in a number of ways. Some biologic effects at high field strengths, such as nerve stimulation and tissue heating, are well understood and have been used to set standards for occupational and public exposure to fields. Other reported effects, particularly at low field strengths, are not as well understood; those include effects on cell metabolism and growth, gene expression, hormones, learning and behavior, and promotion of tumors. The reality of all those effects is the subject of scientific debate and an issue for discussion in this report.



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