Interindividual Variability in Temperature Rise From Localized Exposure at 1--80 GHz Using 131 Anatomical Head Models

Abstract

International exposure guidelines specify the 10-g averaged specific absorption rate (SAR) below 6 GHz and absorbed power density (APD) above 6 GHz as basic restrictions for local exposure. However, quantitative data on interindividual variability in radio frequency (RF)-induced temperature rise remain limited. In this study, temperature rise in the human head was evaluated using 131 anatomical models exposed to near-field RF sources from 1 to 80 GHz. Electromagnetic fields (EMFs) were computed using the finite-difference time-domain (FDTD) method, and steady-state temperature rise was obtained from the bioheat transfer equation. A heating factor (HF), defined as the ratio of the spatial peak temperature rise to 10-g SAR or APD, was used to quantify the relationship between absorbed electromagnetic power and thermal response. Interindividual variability was substantially lower for APD-based HF than for SAR-based HF. The coefficient of variation (CV) remained within approximately 20% for SAR-based HF and below 5% for APD-based HF. Similar behavior was observed across different antenna-to-skin separation distances and skin perfusion conditions. These findings suggest that APD is a stable metric for assessing localized temperature rise above 6 GHz, with only limited influence of anatomical variability.