Numerical Study on Human Brain Cortical Electrostimulation Assessment During Uniform Magnetic Field Exposure at Intermediate Frequencies

Abstract

Permissible limits have been established by international guidelines and standards for human protection against electromagnetic field exposure to prevent adverse health effects stemming from electrostimulation in the most sensitive body part. That is the peripheral nervous system (PNS) in the intermediate frequency range (300 Hz to 100 kHz) and the central nervous system (CNS) at lower frequencies. However, there is a need to reevaluate protection limits against CNS electrostimulation in the intermediate frequency range, considering the importance of brain tissues during electromagnetic head exposure. This study aims to derive the level of CNS cortical stimulation to evaluate compliance with existing protection limits. To achieve this, a numerical multi-scale model was used to evaluate neuron stimulation thresholds by integrating individual neurons into realistic anatomical head models. Five different excitable membrane models within the motor cortex were examined across three human head models, providing the most comprehensive and extensive evaluation to date. The results show that current protection limits are confirmed as conservative, with non-compliance observed in only 0.02% and 2.4% of axons under clamped and sealed boundary conditions, respectively. The study also highlights significant intersubject variability (up to a 600% mean threshold) and clarifies the influence of neural excitation models on permissible level assessments. In conclusion, current electric field limits are conservative for CNS electrostimulation in the intermediate frequency range, but the margin of safety decreases at higher frequencies, warranting further evaluation. The study’s findings and methodology contribute to the rationale and provide valuable insights for re-evaluating electromagnetic safety exposure guidelines.