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 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 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 model 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. The corresponding worst case temporal peak value of the maximum of the Ei averaged over 2 x 2 x 2 mm3 in soft tissue was 2.0 V/m, 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 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.