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 (T-re) 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 degrees C, 75% relative humidity). Mean (+/-SD) ending T-re significantly (P < 0.05) decreased from 38.9 +/- 0.5 degrees C on day 2 to 38.3 +/- 0.4 degrees C on day 7. Likewise, mean (+/-SD) resting T-re significantly (P < 0.05) decreased from 37.0 +/- 0.3 to 36.7 +/- 0.4 degrees C. In fact, all nine men showed a decrease in resting T-re from day 1 to day 7, ranging from -0.1 to -0.5 degrees C. In addition, resting T-re and ending T-re were significantly correlated (r = 0.68). However, the mean increases in T-re tending T-re minus resting T-re) 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 T-re is partially responsible for the attenuation in ending T-re during heat exposure following short-term acclimation to humid heat.