Induced foot-currents in humans exposed to VHF radio-frequency EM fields

In order to limit the specific absorption rate (SAR) in the ankles of a person exposed to an electric field at frequencies below 100 MHz, induced current limits are prescribed in the 1992 ANSI/IEEE safety standard. The authors have measured the induced currents passing through the feet of nine subjects exposed to vertically polarized electric fields from nearby antennas, transmitting at frequencies between 90 and 104 MHz (in the FM broadcast band). The experimental results are in excellent agreement with the results obtained analytically for frequencies up to 110 MHz. The analytical results were obtained by applying the finite difference time domain (FDTD) method to an anatomically-based model of an average height male (1.75 m) and to a model of the tallest subject in the study (1.91 m). For the mean height of the nine subjects (1.75 m), the measured induced foot current for a unit strength vertically polarized electric field varied with frequency from 4.46 to 3.45 mA/(V/m) for frequencies between 90-104 MHz. For the tallest subject (1.91 m), the corresponding values ranged from 5.42 to 4.45 mA/(V/m), Foot currents in excess of the induced current limits in RF safety guidelines for both the controlled and uncontrolled environments could result even when the vertical component of the incident electric fields comply with the corresponding field limits. It is important, therefore, to not only measure the E- and H-fields, but also the induced currents up to the recommended maximum frequency of 100 MHz, and perhaps up to the upper frequency of the FM broadcast band (108 MHz)