Far-infrared sauna exposure at 65°C elevates core temperature

Far-infrared (FIR) saunas are increasingly used as a passive heating and 'wellness' modality, yet their capacity to elevate core temperature remains unclear, particularly given prior work at lower temperatures (∼45°C) reports limited net heat storage. We therefore examined whether FIR sauna exposure at an ambient temperature more typical of traditional sauna bathing (65°C) increases core (rectal) temperature and evokes thermoregulatory, cardiovascular, haematological and perceptual responses associated with heat strain. Twelve healthy adults (6 female) completed a single 45-min FIR sauna exposure. Rectal temperature, skin temperature, heart rate, blood pressure, plasma volume changes and perceptual measures were assessed throughout. Rectal temperature increased by 1.4°C (95% CI: 0.9, 1.8; P < 0.001), with elevations evident from ∼20 min. Skin temperature rose rapidly (+7.4°C [95% CI: 5.4, 9.5] at 5 min; P < 0.001) and remained elevated thereafter. Heart rate increased from 74 ± 15 to 153 ± 20 bpm (P < 0.001), while mean arterial pressure decreased immediately post-exposure (-9 mmHg [95% CI: -2, -16]; P = 0.012), driven by reductions in diastolic pressure. Plasma volume contracted by -11.6% (95% CI: -15.9, -7.2; P < 0.001), with a sweat rate of 1.4 ±  0.5 L h^{- 1}. Thermal sensation and discomfort increased to 'extremely hot' and 'extremely uncomfortable', respectively (P < 0.001). In summary, FIR sauna exposure at 65°C elicits substantial increases in rectal temperature and systemic physiological strain comparable to values previously reported in the passive heating literature. These findings indicate that the effectiveness of FIR sauna to increase core temperature is governed by the thermal load imposed, supporting its potential as a heat-based intervention when applied at sufficiently high temperatures.