Effects of dry and mist saunas on circulatory and thermoregulatory functions in humans

To test the hypothesis that mist sauna is a safer way of bathing than dry sauna, we compared changes in circulatory and thermoregulatory functions during 10 min sauna bathing in mist sauna at 40 ̊C with relative humidity of 100%, and in dry sauna by infrared ray at 70 ̊C with relative humidity of 15%. Subjects were seven healthy young men aged 29 ± 6 yrs (mean ± SD). We measured blood pressure, heart rate, skin temperatures at chest, forearm, thigh, and leg, tympanic temperature (Tty) by thermistors, skin blood flow at forearm by laser Doppler flowmetry, and sweat rate by ventilated capsule method at 1 min intervals throughout the experiment. Total sweating and change of hematocrit were also measured for dehydration analysis. Blood pressure was elevated more and changes in heart rate and total sweating were larger in dry sauna than mist. A significant hematocrit increase was observed in dry sauna bathing only. Mean skin temperature and Tty in dry sauna were elevated higher than those in mist. Heat stress of the dry sauna may be stronger than that of the mist, leading to dehydration and hypovolemia by sweating. Percent plasma volume loss was significantly larger in the dry than mist sauna. Changes in skin blood flow and sweat rate/∆Tty during mist sauna were significantly larger than those during dry sauna bathing despite heat stress of the mist sauna. The mist sauna bathing may thus be safer physiologically, and provide more effective vascular dilatation and sweating than the dry sauna bathing.