Heat of sorption induced by sweating affects thermoregulatory responses during heat load

To investigate the effects of heat of sorption per se induced by sweating on thermoregulatory responses in clothed subjects, two experiments were carried out. In experiment 1 [ambient temperature (Ta) 27.2 degrees C, 50% relative humidity (r.h.)], seven female subjects immersed their lower-legs in a water bath at a temperature raised between 35-41 degrees C during 70 min wearing garments made for the experiment of either 100% cotton (C) or 100% polyester (P). Skin blood flow at the forearm was significantly greater in C than in P (P < 0.05). The increase of mean skin temperature (Tsk) and clothing surface and clothing microclimate temperatures were significantly higher in C than that of P after the onset of sweating (P < 0.05). Furthermore, these responses were accompanied by warmer and more uncomfortable sensations in C than in P despite a lower rectal temperature in C compared with P and identical mean body temperature in both sets of garments. In experiment 2, to simulate the clothing microclimate after the onset of sweating, C and P garments were exposed in the climate chamber to r.h. raised from 50% to 95% at a constant Ta of 27.2 degrees C. The clothing surface temperature rose by 2.2 degrees C for C and by 0.5 degree C for P after the increase of r.h. These results clearly showed a more marked increase in heat of sorption in C than in P. These results indicated that the heat of sorption per se after the onset of an increasing clothing microclimate vapour pressure, mimicking the onset of sweating, enhanced thermoregulatory responses such as skin blood flow, (Tsk) and subjective voting in C.