Intensity-dependent Temperature Rise Induced by Local Exposure to 26.5 GHz Quasi-Millimeter-Wave in Rat

The widespread use of fifth-generation 5G millimeter-waves (MMW) generates concern about potential adverse health effects. The latest international guidelines for MMW exposure adopt an absorbed power density (APD) of 200 W/m² to avoid a local temperature rise of 5°C in human tissues as an operational adverse health effect threshold. However, because APD is estimated by simulations using human tissue models, it is unknown whether a similar value can be confirmed for living tissues. The aim of this study was to investigate the relationship between APD and skin temperature rise in vivo, and to validate the estimated values.

The rat dorsal skin was locally exposed to a 26.5 GHz quasi-MMW (qMMW) for 18 min using a patch antenna. The qMMW exposure intensities estimated by dosimetry were set to 0-500 W/m² of APD. The temperatures in the dorsal skin and rectum were simultaneously measured during exposure.

The qMMW-induced local temperature increase at different sites. The dorsal skin temperature increased by approximately 11.3°C at a maximum intensity of 500 W/m², but the rectal temperature increased by only 0.6°C, indicating highly localized effects of exposure to rats. A significant correlation was observed between APD and skin temperature rise. The relationship provided a linear regression model, and a temperature rise of less than 5°C was estimated in the skin exposed to 200 W/m² of APD.

These results suggest that the operational threshold for the MMW exposure guidelines is valid under the present experimental conditions using rats.