Evaluation of Nonuniform 60-Hertz Magnetic-Field Exposures for Compliance with Guidelines

Abstract

Magnetic-field exposures are considered in compliance with guidelines if they do not cause the induced electric field or current density to exceed basic restrictions that are based on possible adverse biological responses. Magnetic-field guidelines provide induction models for extrapolating from external field exposures to basic restrictions and vice versa. However, the uniform-field exposures used in these models do not reflect the nonuniform fields often encountered in actual high-field exposures. The purposes of this study were to investigate the relationships between external magnetic-field exposures and induced electric fields in nonuniform 60-hertz fields and to present a method for evaluating the compliance of such exposures with guidelines. Induction factors provide the induced electric field per unit of incident magnetic field. They represent a means of extrapolating from external field exposure to a peak induced electric field. Uniform and nonuniform field induction factors were computed for homogeneous ellipses and ellipsoids, and for an anatomically correct heterogeneous human model. Computations were carried out for three orthogonal uniform fields and for related nonuniform fields at varying distances from three line sources. Analytic expressions were used to compute induced peak electric fields for homogeneous models in uniform fields. A scalar-potential finite-difference method computed induced quantities for all models at 3.6-mm resolution in uniform and nonuniform fields. Equivalent uniform magnetic fields that produce the same peak electric field as a nonuniform field with a known maximum field were derived from the induction factors. To evaluate a nonuniform field exposure for compliance, the equivalent uniform field for the exposure is estimated based on the magnitude of the maximum surface field and the distance from the line source. Compliance is achieved if the equivalent uniform magnetic field is below the magnetic-field limit. Equivalent uniform magnetic-field exposures are computed for two actual electric utility tasks, as examples.

Recent advances in research relevant to electric and magnetic field exposure guidelines

“…Similar SPFD computations were performed for uniform and non-uniform magnetic field exposures of a heterogeneous, anatomically correct human model [Bracken and Dawson, 2004]. Again, normalized induction factors were generated for the human model as a function of distance from a single-line source for three orthogonal field orientations.…”

“…Computations of the induced electric fields and currents from non-uniform magnetic fields in the homogeneous conductive models that are used in exposure guidelines are now available [Bracken, 2002;Bracken and Dawson, 2004;IEC, 2004]. Computations of induction by non-uniform fields in anatomically correct human models with heterogeneous tissue characteristics have also been reported [Bracken and Dawson, 2004].…”

“…Computations of induction by non-uniform fields in anatomically correct human models with heterogeneous tissue characteristics have also been reported [Bracken and Dawson, 2004]. The results of these computations suggest a method for ascertaining compliance with exposure guidelines when occupational exposures to magnetic fields in excess of guideline reference levels occur [ICNIRP, 1998;IEEE/ICES, 2002].…”

“…Induced quantities for the ellipsoids in uniform and non-uniform fields have also been computed with a SPFD method [Bracken and Dawson, 2004]. A 60-Hz single-line source was positioned to produce asymptotical uniform fields in the sagittal, coronal, and vertical orientations ( Table 5).…”

Dosimetric Uncertainties

“…The value of C PN is unique to one's body and tissue dimensions: the relative conductivities of adjoining tissues for frequencies below 100 kHz (permittivity is included at higher frequencies); anisotropy of tissue; for excitable tissue, the electrostimulation threshold unique to a specific site in a specific individual; the location of the applicable sensory receptor within its tissue; an individual's processing of sensory information in the CNS; the orientation and uniformity (or non-uniformity) of the exposure field relative to the individual; and likely other factors not mentioned. Bracken and Dawson introduced the concept of "induction factors" to normalize induced electric fields and current densities to external magnetic fields in the case of exposure to non-uniform 60-Hz magnetic fields (Bracken and Dawson 2004).…”