Alternating magnetic fields generate electric current in any conducting medium. If the properties of the tissue are constant, the induced current is directly proportional to the frequency of the field. At very high frequencies or very high amplitudes, induced currents can heat biological tissue, causing thermal damage. At extremely low frequencies (ELF -- approximately 0-300 Hertz) and very low frequencies (VLF - approximately 300-100,000 Hertz), tissue heating is not a problem, but if induced currents are too strong, there is a risk that they may stimulate electrically excitable cells such as neurons. Health effects not linked to cell stimulation or tissue heating have been reported, but only a few of these effects have been demonstrated scientifically, and only at extremely low frequencies.
At frequencies above 100kHz, the currents induced by electromagnetic fields can heat biological tissue. Heating of biological tissue can damage it; for example, the role of tissue heating in cataract development is well documented (IRPA, 1988). Tissue heating is also readily detectable by the animal, and even very low levels of radio frequency induced tissue heating will sometimes cause a non-human primate to stop whatever it's doing, in order to ``cool off.'' The IEEE/ANSI standard for exposure to alternating magnetic fields above 100kHz is therefore designed to maintain the field strength a factor of ten below the level which would cause disruption of ongoing behavior in non-human primates (IEEE, 1992).
At frequencies below approximately 100kHz, the current necessary to significantly heat biological tissue is greater than the current necessary to stimulate neurons and other electrically excitable cells. At power line frequencies (50-60 Hertz), excitable cells (e.g. neurons) can be stimulated by current densities of 10-100 uA/cm² (IRPA, 1990). At higher frequencies, the threshold for stimulation of excitable cells increases; between 3kHz and 100kHz, the lowest reported thresholds for cell stimulation are greater than 35f uA/cm², where f is expressed in kilohertz (IEEE, 1992). The IEEE/ANSI standard limit on exposure to alternating magnetic fields between 3kHz and 100kHz is designed to keep induced currents at least a factor of ten below the lowest reported thresholds for stimulation of excitable cells.
At amplitudes which are too low to stimulate excitable cells, ELF electric and magnetic fields (specifically, 60Hz fields) have been conclusively linked to a small number of health effects, and other health effects have been suggested but not proven. According to the review by Stuchly (1995), exposure of healthy male volunteers to 20uT electrical and magnetic fields at 60 Hz has been linked to a statistically significant slowing of the heart rate, and to changes in a small fraction of the tested behavioral indicators. There is evidence that nocturnal exposure to 60Hz magnetic fields affects melatonin production. Case studies have suggested that exposure to ELF electromagnetic fields may promote the growth of cancer, but laboratory tests involving rodents have been almost entirely negative.