Temperature and humidity independently influence thermoregulatory responses during Finnish sauna bathing

This study examined the effects of sauna air temperature and relative humidity (RH) on acute cardiovascular and thermoregulatory responses under typical Finnish sauna conditions, where participants were free to throw water onto the heated stones (löyly). Fifty recreationally active healthy adults (32 males, 18 females, 35.4 ± 10.4 years, 174 ± 9.9 cm, 77 ± 17 kg) completed four 10-minute sauna sessions, each separated by 30-minute recovery. Core temperature (T_core), skin temperature, heart rate (HR), body mass, and perceptual responses were measured, alongside detailed multi-point environmental monitoring of temperature and relative humidity (RH) monitoring. Generalized estimating equations assessed associations between environmental factors and physiological responses. HR increased markedly during sauna exposure (mean +41.6 bpm), while T_core rose moderately (+0.4°C). Temperature and RH were independently associated with increases in HR (β = 1.40 bpm/°C; β = 0.37 bpm/%RH) and T_core (β = 0.032°C/°C; β = 0.0079°C/%RH). These associations remained significant after adjustment for sex, physical activity, body fat percentage, and skin surface area. The difference between dew point and skin temperature, used as an index of condensation potential, showed weak associations with HR and no significant relationship with T_core. Repeated sauna sessions did not meaningfully influence HR responses, and only minor differences were observed in T_core between sessions. Perceptual responses were primarily driven by temperature, with smaller contributions from humidity. These findings demonstrate that humidity, beyond temperature, appears to independently contribute to physiological strain during sauna exposure. The results highlight the importance of characterizing environmental conditions and suggest that humidity should be considered when optimizing sauna use for potential health benefits.