Dosimetric Uncertainties: Magnetic Field Coupling to Peripheral Nerve.

Abstract

The International Commission on Non-ionizing Radiation Protection (ICNIRP) and the Institute for Electrical and Electronic Engineers (IEEE) have established magnetic field exposure limits for the general public between 400 Hz (ICNIRP)/759 Hz (IEEE) and 100 kHz to protect against adverse effects associated with peripheral nerve stimulation (PNS). Despite apparent common purpose and similarly stated principles, the two sets of limits diverge between 3.35-100 kHz by a factor of about 7.7 with respect to PNS. To address the basis for this difference and the more general issue of dosimetric uncertainty, this paper combines experimental data of PNS thresholds derived from human subjects exposed to magnetic fields together with published estimates of induced in situ electric field PNS thresholds to evaluate dosimetric relationships of external magnetic fields to induced fields at the threshold of PNS and the uncertainties inherent to such relationships. The analyses indicate that the logarithmic range of magnetic field thresholds constrains the bounds of uncertainty of in situ electric field PNS thresholds and coupling coefficients related to the peripheral nerve (the coupling coefficients define the dosimetric relationship of external field to induced electric field). The general public magnetic field exposure limit adopted by ICNIRP uses a coupling coefficient that falls above the bounds of dosimetric uncertainty, while IEEE's is within the bounds of uncertainty toward the lower end of the distribution. The analyses illustrate that dosimetric estimates can be derived without reliance on computational dosimetry and the associated values of tissue conductivity. With the limits now in place, investigative efforts would be required if a field measurement were to exceed ICNIRP's magnetic field limit (the reference level), even when there is a virtual certainty that the dose limit (the basic restriction) has not been exceeded. The constraints on the range of coupling coefficients described in this paper could facilitate a re-evaluation of ICNIRP and IEEE dose and exposure limits and possibly lead toward harmonization.