Single exposure to near-threshold 5G millimeter wave modifies restraint stress responses in rats

Authors: Matsumoto A, Endo I, Ijima E, Hirata A, Kodera S, Ichiba M, Tokiya M, Hikage T, Masuda H

Year: 2025

Category: Toxicology

Journal: Environ Health Prev Med

DOI: 10.1265/ehpm.24-00321

URL: https://pmc.ncbi.nlm.nih.gov/articles/PMC12062831/

Abstract

Background

Concerns have risen regarding the health effects of quasi-millimeter waves (qMMW) utilized in 5th-generation wireless communication systems. Guidelines have conservatively set a whole-body average specific absorption rate (WBA-SAR) of 4 W/kg—equivalent to a 1°C increase in core temperature—as the operational threshold for adverse effects. However, direct biological evidence for these thresholds has been lacking. Previous experiments showed a 1°C core temperature increase following 30 minutes of qMMW exposure at 4.6 W/kg in rats, highlighting the need for further investigation into biological stress responses near this threshold.

Methods

  • Fifty-nine young Sprague-Dawley rats (240-322 g) were exposed to 28 GHz for 40 minutes at WBA-SARs of 0, 3.7, and 7.2 W/kg.
  • Experiments were conducted under normal (22.5°C, 45-55% humidity) and heat (32°C, 70% humidity) conditions, with rats restrained in acrylic holders.
  • Serum and urinary biomarkers of stress were repeatedly measured and analyzed using mixed models, accounting for sham exposure and between-subject variation.

Key Findings

  • Sham exposure induced stress responses, primarily due to restraint stress.
  • After adjusting for sham effects, 28 GHz exposure elevated serum-free corticosterone 1 or 3 days post-exposure, especially in animals with >1°C rectal temperature change.
  • Urinary-free catecholamines indicated that 28 GHz exposure has an inhibitory effect on the stress response (notably noradrenaline) immediately after exposure.
  • Heat exposure amplified the effect, with a correlation between noradrenaline and tail surface temperature, supporting the role of noradrenaline in heat dissipation.

Conclusions

This study provides the first direct evidence that qMMW whole-body exposure near threshold values can modify the stress response in rats, as indicated by changes in corticosterone and noradrenaline. These findings offer biological insight into whole-body exposure thresholds specified by international safety guidelines and underscore the potential stress-related health risks associated with near-threshold 5G millimeter wave exposures.

5G millimeter wave stress response SAR corticosterone noradrenaline rat study
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