fields are coupled, but in the limit of unchanging fields, they become independent. For frequencies encountered in electric-power transmission and distribution, these two fields can be considered independent to an excellent approximation. For extremely-low-frequency fields, including those from power lines and home appliances and wiring, the electric component is easily attenuated by metal elements in residential construction and even by trees, animals, and people. The magnetic field, which is not easily attenuated, is generally assumed to be the source of any possible health hazard. When animal bodies are placed in a time-varying magnetic field (as opposed to remaining stationary in the earth's static magnetic field), currents are induced to flow through tissues. These currents add to those that are generated internally by the function of nerve and muscle, most notably currents detected in the clinically useful electroencephalogram and the electrocardiogram. The currents produced by nerve and muscle action within the body have no known physiologic function themselves but rather are merely a consequence of the fact that excitable tissue (such as nerve and muscle) generate electric currents during their normal operation.
General conclusions from the review of the literature involving studies of exposure assessment and the physical interactions of electric and magnetic fields with biologic systems are the following:
Exposure of humans and animals to external 60-hertz (Hz) electric and magnetic fields induces currents internally.
The density of these currents is nonuniform throughout the body. The spatial patterns of the currents induced by the magnetic fields are different from those induced by the electric fields. Electric fields generally are measured in volts per meter and magnetic fields in microtesla (µT) or milligauss (mG) (1 µT = 10 mG).
Ambient levels of 60-Hz (or 50-Hz in Europe and elsewhere) magnetic fields in residences and most workplaces are typically 0.01-0.3 µT (0.1-3 mG).
Higher levels are encountered directly under high-voltage transmission lines and in some occupational settings. Some appliances produce magnetic fields of up to 100 µT (1 G) or more in their vicinity. For comparison, the static magnetic field of the earth is about 50 µT (500 mG). Magnetic fields of the magnitude found in residences induce currents within the human body that are generally much smaller than the currents induced naturally from the function of nerves and muscles. However, the highest field strengths to which a resident might be exposed (those associated with appliances) can produce electric fields within a small region of the body that are comparable to or even larger than the naturally occurring fields, although the magnitude of the largest locally induced fields in the body is not accurately known.
Human exposure to a 60-Hz magnetic field at 0.1 µT (1 mG) results in the maximum current density of about 1 microampere per square meter (µA/m2).
The endogenous current densities on the surface of the body (higher densities occur internally) associated with electric activity of nerve cells are of the order