Measuring Multifrequency Magnetic Fields for Exposure Assessment Based on Induced Body Currents

Standards setting committees in the United States and Great Britain have adopted exposure limits for magnetic fields with a single frequency in the sub-radio frequency (sub-RF) range (1 Hz to 30 kHz). Since sub-RF magnetic fields in most occupational and residential settings have multiple frequencies, a more practical exposure assessment approach is derived in this article. To avoid adverse health effects, the present exposure limits control the currents induced in the body by the magnetic field. In this article, limits on induced current are shown to be equivalent to placing limits on the magnetic field's derivative dB/dt, both of which are applicable to the multifrequency fields found in the environment. The existing exposure limits are therefore used to derive bounds on dB/dt. Commercially available gaussmeters can be used to measure sub-RF magnetic fields for comparison with these limits on dB/dt exposures. To measure dB/dt, the gaussmeter needs a linear frequency response, and a correction factor must be applied to the readout. The errors in assessing induced currents with multiple frequencies are evaluated for other common methods of measuring magnetic fields. These other methods can have acceptable accuracy, but only if the field's harmonic distortion is sufficiently low. Practical methods are recommended for measuring multifrequency magnetic fields and assessing their induced body currents. The relationship between sub-RF magnetic fields and adverse health effects is not addressed in this article.