Acclimation to humid heat lowers resting core temperature.

The purpose of this study was to test the hypothesis that a reduction in resting rectal temperature (Tre) is partially responsible for the attenuation in the rise of core temperature during heat exposure following acclimation to humid heat. Nine male volunteers completed 7 days of acclimation, performing 2 h of exercise per day in a hot, humid environment (35°C, 75% relative humidity). Mean (±SD) ending Tre significantly ( P < 0.05) decreased from 38.9 ± 0.5°C on day 1 to 38.3 ± 0.4°C on day 7. Likewise, mean (±SD) resting Tre significantly ( P < 0.05) decreased from 37.0 ± 0.3 to 36.7 ± 0.4°C. In fact, all nine men showed a decrease in resting Tre from day 1to day 7, ranging from -0.1 to -0.5°C. In addition, resting Tre and ending Tre were significantly correlated ( r = 0.68). However, the mean increases in Tre (ending Tre minus resting Tre) and heat storage that occurred on each of the 7 acclimation days were not significantly different. These results support the hypothesis that a reduction in resting Tre is partially responsible for the attenuation in ending Tre during heat exposure following short-term acclimation to humid heat.