Temperature elevation in the human body model for RF plane-wave exposure

Elevated temperatures (1-2°C) resulting from radio frequency (RF) absorption can cause adverse health effects, such as heat exhaustion and heat stroke. In the International Commission on Non-Ionizing Radiation Protection (ICNIPR) guidelines [1] and the IEEE standard [2], the whole-body-averaged specific absorption rate (WBA-SAR) is used as a metric of human protection from RF whole-body exposure. In these guidelines, the basic restriction of WBA-SAR is 0.4 W/kg for occupational exposure and the reduction factor of five is applied for general public exposure. This threshold is based on the fact that RF exposure of laboratory animals in excess of approximately 4 W/kg has revealed a characteristic pattern of thermoregulatory response [3]. For localized exposure, the peak value of spatial SAR averaged over 10 g of tissue is used as a metric. The limit is 10 W/kg for occupational exposure and 2 W/kg for general public in the standard/guideline [1, 2]. These limits have been set with the objective of preventing local temperature elevation in heat sensitive tissue [2]. However, the upper limits of the frequency ranges where the local and WBA-SARs are applied are different. Specifically, the upper frequencies for WBA- and local SARs are both 10 GHz in the ICNIRP guidelines [1], while they are 6 GHz and 3 GHz in the IEEE standard [2]. In the radio-wave law in Japan, the upper frequency was changed from 3 GHz to 6 GHz. According to the IEEE standard [2], the upper boundary of the frequency range over which WBA-SAR is deemed to be the basic restriction has been reduced from 6 GHz to 3 GHz because radio-wave energy is absorbed around the body surface with the increase of the frequency. However, no rationale is given for supporting that description. It is of interest to investigate the maximum temperature elevation in addition to the core temperature even for far-field exposure to clarify the rationale of the standard from the engineering/physics standpoint.