Permissible SA and Radiant Exposure for Brief Exposure in GHz Region

The specific absorption rate (SAR) averaged over 10 g of tissue is used as a physical quantity to prevent local temperature elevation in the international guidelines for constant exposure to radio-frequency electromagnetic fields. The averaging time of SAR is set to 6 min for local exposure. The ICNIRP public consultation document introduced a new physical quantity, absorbed power density (APD), as the basic restriction above 6 GHz. In addition, the limit for the specific absorption (SA) and absorbed energy density (AED) were set to avoid heating for brief exposures (shorter than the averaging time of 6 min). If all the energy for the averaging time of 6 min for continuous wave are concentrated into short duration and multiple pulse exposures, the local temperature elevation for the pulse(s) may exceeds that of exposure at almost constant exposure although it satisfies the limit for the SAR/APD. The purpose of this study is to determine how best to set the metric in order to limit temperature rise from brief exposures. We first confirmed that the peak temperature elevation for brief pulse exceeds the steady-state temperature for continuous exposure. We then proposed the limit energy density from the regression curve depending on the exposure duration of brief-pulse(s). The time course of temperature elevation in the multi-layer cube model was computed for typical scenarios of multiple pulses exposures. The SA and AED based on the regression curve can be used as a metric to prevent excess temperature elevation for different brief exposure scenario.