for the average electric field, exposures to 14-18-V/m electric fields and 0.1-µT (1-mG) magnetic fields are equivalent, and for the maximum induced electric fields, 3-V/m electric fields and 0.1-µT (1-mG) magnetic fields are equivalent. In comparing the maximum values of either the induced current density or the electric fields, very close environmental levels of electric (4 V/m and 3 V/m) and magnetic (0.1 µT) fields are obtained.

Endogenous current densities associated with action potentials of excitable tissues are of the order of 1 mA/m2 or an electric field of approximately 1 mV/m. To obtain similar induced current densities from exposure to external 60-Hz fields, human exposures of about 2-kV/m electric fields or 100-µT (1-G) magnetic fields would be required. Those fields are considerably larger than are commonly encountered in the residential environment.

The induced currents and fields have been evaluated so far for a grossly simplified structure of tissues by considering only its bulk electric properties. Inclusion of cellular structure, including anisotropies, presents a formidable task, so far unsolved (McLeod 1992; Polk 1992a,b).

Evaluation of induced current and electric fields is also important in quantifying and interpreting results of in vitro laboratory studies. It is especially important when determining whether the biologic effect observed is due to the magnetic field or the electric currents and fields induced in the test sample by the magnetic field. When results of a study in one laboratory are not corroborated by other data from other laboratories, evaluation of induced fields might also be useful in finding the differences in apparently identical experiments.

For low-density biologic cells placed in a conductive medium, the induced current density can be computed solely on the basis of geometry of the medium contained in the exposure dish and the magnetic-field characteristics (Misakian et al. 1993). Methods of calculation for several dish shapes, including an annular ring, have been published (McLeod et al. 1983; Misakian and Kaune 1990; Misakian 1991; Misakian et al. 1993; Wang et al. 1993). Some dish configurations and magnetic-field orientations facilitate obtaining the same current density in most of the medium volume occupied by cells. However, even at low densities, the presence of biologic cells affects the spatial pattern of the induced currents and fields because of the low conductivity of the cell membranes. The effect of cell density is much more pronounced when density is high and when cells form a confluent monolayer (Hart et al. 1993; Stuchly and Xi 1994).

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