Pain threshold induced by local exposure to millimeter waves on human skin

Exposure to millimeter waves potentially causes pain and tissue damage at the exposed body sites. However, the threshold temperature, especially the pain threshold, remains unclear. In this study, we investigated the perception thresholds of four different types of sensations-warmth, heat, tingling, and pain-and individual physical characteristics associated with the pain perception induced by millimeter-wave exposure. Thirty-five healthy adults were exposed to a 28 GHz millimeter wave on their left middle fingertip at an input power of 8.5 W (incident power density averaged over a 1 cm² area: 1.00 W/cm²) for up to 10 min. The temperature at which the initial subjective perception of each of the four sensation types occurred was determined as the perception threshold for that sensation. When the skin surface temperature exceeded 44°C or when the participant felt pain, the exposure was terminated. The association between participants' physical characteristics and the presence of pain was investigated using logistic regression analysis. Consequently, the perception thresholds increased in the following order: warmth < heat < tingling < pain. A total of 18 (51%) participants perceived pain, and the average pain threshold was 41.9°C. Females were more likely than males to perceive pain (odds ratio, 4.80 [1.15-20.09]), which may be partially explained by sex-related differences in hydration levels and finger circumference. Our results demonstrated that exposure to millimeter waves can induce pain at approximately 42°C and revealed clear sex differences in sensitivity to pain perception under millimeter-wave exposure.