Dosimetric analysis of hands exposure during handling of strong permanent magnets

Abstract

Workers in a production line for synchronous motors occasionally reported tingling sensations or a feeling of numbness in their hands when handling strong permanent magnets. As the magnetic flux density (B) and its gradients along and close to the surface of the permanent magnets were expected to be comparably high and the movements of the workers’ hands may therefore cause relevant induction inside the tissue, a detailed dosimetric analysis of the in situ electric field inside the hands (Ei) of the workers was carried out. The time derivate of the magnetic flux density (dB/dt) occurring along the hands was determined based on time domain measurements using a specially developed ‘measurement glove’ containing 12 Hall sensors. Based on these measurement results temporal peak electric field strength (Ei) induced inside a newly developed high resolution anatomical hand models were numerically computed, using the scalar potential finite difference (SPFD) method. The highest measured dB/dt along the palmar side of the hand was 51.2 T s−1. The corresponding worst case temporal peak value of the maximum of the Ei averaged over 2 × 2 × 2 mm3 in soft tissue was 2.0 V m−1, which is a factor of 1.8 higher than the applicable exposure limit value, but still below the range of 3.8–6.2 V m−1 which is presently assumed the range of lowest stimulation threshold for peripheral nerves. Our analysis did therefore not provide an indication that the perception reported by the workers are due to tissue stimulation in the sense of provoking action potentials.

Numerical assessment of induced electric fields in a worker’s hand with commonly used metallic implants under exposure to low frequency magnetic fields

“…The anatomical hand/forearm model used is an extended version of the hand model described in David and Schmid (2020). The model is based on MRI scans of an adult hand and forearm.…”

“…The procedure applied during the MRI scans followed the protocol described in Überbacher et al (2019). For the work described here, the original hand model was extended approximately 20 cm towards the elbow using the workflow described in David and Schmid (2020). Moreover, the hand size was scaled to approximately fit the 95th percentile of male hands (White 2.4.…”