The reliability of a portable steam sauna pod for the whole-body passive heating of humans

Introduction: Passive heating is receiving increasing attention within human performance and health contexts. A low-cost, portable steam sauna pod may offer an additional tool for those seeking to manipulate physiological (cardiovascular, thermoregulatory and sudomotor) and perceptual responses for improving sporting or health profiles. This study aimed to 1) report the different levels of heat stress and determine the pods’ inter-unit reliability, and 2) quantify the reliability of physiological and perceptual responses to passive heating. Method: In part 1, five pods were assessed for temperature and relative humidity (RH) every 5 min across 70 min of heating for each of the 9 settings. In part 2, twelve males (age: 24 ± 4 years) completed two 60 min trials of passive heating (3 × 20 min at 44 °C/99% RH, separated by 1 week). Heart rate (HR), rectal (T _rectal) and tympanic temperature (T _tympanic) were recorded every 5 min, thermal comfort (T _comfort) and sensation (T _sensation) every 10 min, mean arterial pressure (MAP) at each break period and sweat rate (SR) after exiting the pod. Results: In part 1, setting 9 provided the highest temperature (44.3 ± 0.2 °C) and longest time RH remained stable at 99% (51±7 min). Inter-unit reliability data demonstrated agreement between pods for settings 5–9 (intra-class correlation [ICC] >0.9), but not for settings 1–4 (ICC <0.9). In part 2, between-visits, high correlations, and low typical error of measurement (TEM) and coefficient of variation (CV) were found for T _rectal, HR, MAP, SR, and T _comfort, but not for T _tympanic or T _sensation. A peak T _rectal of 38.09 ± 0.30 °C, HR of 124 ± 15 b min ⁻¹ and a sweat loss of 0.73 ± 0.33 L were reported. No between-visit differences (p > 0.05) were observed for T _rectal, T _tympanic, T _sensation or T _comfort, however HR (+3 b.min ⁻¹) and MAP (+4 mmHg) were greater in visit 1 vs. 2 (p < 0.05). Conclusion: Portable steam sauna pods generate reliable heat stress between-units. The highest setting (44 °C/99% RH) also provides reliable but modest adjustments in physiological and perceptual responses.