Changes in the index of sweat ion concentration with increasing sweat during passive heat stress in humans

To investigate the pattern changes in the index of sweat ion concentration at skin surface with increasing sweat during passive heat stress in humans, we measured conductivity of the perfused water with sweat as the index of sweat ion concentration and sweat rate, continuously at the chest skin surface. Eight healthy subjects (22.4 ±1.0 years) were passively heated by lower-leg immersion in a hot water bath of 42°C for 50 min in an ambient temperature of 28°C and relative humidity of 50%. The internal temperature (Tor) thresholds of sweat rate and index of sweat ion concentration were almost similar. Concomitant onset for the index of sweat ion concentration and sweat rate occurred but two types of linear regression lines were identified in the relationship between the index of sweat ion concentration and sweat rate at a boundary sweat rate value of 0.30 ± 0.08 mg cm^–2 min^–1. The slope of the regression line at low levels of sweat (slope 0.02 ± 0.01 V mg^–1 cm^–2 min^–1) was significantly gradual compared with that at moderate levels of sweat (slope 0.30 ± 0.08 V mg^–1 cm^–2 min^–1) (P<0.05). These results suggest that at low levels of sweat the index of sweat ion concentration responds gradually with respect to sweat rate, which may be due to the ion reabsorption capacity of the sweat duct, and then the index of sweat ion concentration increased steeply with sweat rate.