Thresholds for biological effects of time-varying magnetic fields.

Investigations of the biological effects of time-varying magnetic fields in the extremely low frequency range have been carried out with particular reference to NMR imaging. One effect encountered is the stimulation of nerves and muscles by induced eddy currents. The nature of this effect has been studied in rats exposed to whole body homogeneous fields and in the human forearm with local exposures. Electromyography was used to provide quantitative measurements of thresholds and responses for human exposures. The field exposures required to produce a detectable response are dependent upon the pulse period, pulse waveform and the current path in tissue. For the rat experiments the lowest threshold occurred for a damped sinusoidal pulse of period 0.33 ms with a maximum rate of change of field of 2400 T s-1. The lowest threshold for sensation in the human experiments was 2100 T s-1 for a similar type of field. Simple theory for conduction is considered and current densities induced are estimated as 5 A m-2.