Heat acclimatization, its decay and reinduction in young Caucasian females

The loss of heat acclimatization and the number of days necessary for reinduction to the acclimatized state as a function of length of the decay period were studied for periods of four, eight, and twelve days. Six females between 18 and 30 years of age (mean VO2 max = 38.7 mL/kg/min) were concurrently heat acclimatized for ten days by treadmill walking at 5.0 km/hr for 100 minutes per day in an environment of 33.5 degree C WBGT. Both 35% and 75% relative humidities were utilized to achieve at 33.5 degree C WBGT with half the subjects exposed to each condition. Physiological measurements included heart rate, sweat rate, core temperature, and mean skin temperature. Of these measurements, only the sweat rate proved not to be affected by any of the independent variables. Predictive equations were developed for heart rate, core temperature and mean skin temperature for decay periods of four through twelve days in length and end of work values attainable for one through four days of successive work after decay. Significant relationships were found in all cases, with the correlation coefficients ranging from 0.73 to 0.86. Both levels of humidity created the same physiological responses from the subjects, suggesting that the ability of WBGT to evaluate heat stress is independent of the humidity level.