Studies on heat tolerance at rest and orthostatic heat tolerance in a hot environment.
Twelve young male Japanese were exposed to a hot environment in summer, and changes in systolic blood pressure induced by changing the position from recumbent to sitting were observed. Sweating was induced in a climatic chamber at 30 degree C with 70% relative humidity by immersing both legs up to the knees into a water bath of 42 degree C for 90 min after sitting on a chair at rest for 30 min in the chamber. The mean values of rise in rectal temperature, body weight loss and mean sodium concentration in sweat were 0.62 degree C, 0.67 kg and 46 mEq/l, respectively. The mean values of fall in systolic blood pressure associated with changing posture and its recovery time were 13.7 mm Hg and 77.5 sec, respectively. Heat tolerance at rest was assessed by a numerical heat tolerance index involving relative water loss, relative rise in rectal temperature, and relative salt loss. Values of relative water loss, relative rise in rectal temperature, and the numerical heat tolerance index correlated closely to those of fall in systolic blood pressure and its recovery time.