Tolerance of Very Hot Humid Environments by Highly Acclimatized Bantu at Rest

In order to know as much as possible about human reactions under conditions of extreme ambient air temperatures, and about the ability of men to survive under these conditions and to assist in their own survival, a series of studies was made in which groups of 10 men were exposed to temperature conditions ranging from 96°F. to 104°F. wet bulb (W.B.). Rigid criteria were employed to decide when men should be withdrawn from the test conditions. These were: (a) reaching a rectal temperature of 104°F., (b) suffering from repeated attacks of heat collapse which prevented the subject from standing erect, and (c) suffering a change in temperament and being no longer susceptible to instructions.

At W.B. temperatures of 104°F. and 102°F. the rectal temperatures of the men continued to rise in a straight line until they reached 104°F. There was no tendency for the rectal temperatures to reach a steady state. At 100°F. W.B. there was a definite tendency in the rectal temperatures towards a steady level, but a steady state was not reached until the rectal temperatures had reached 104°F. The same general trend was seen at 98°F., but the time taken to reach a steady state was increased considerably. At 96°F. the rectal temperatures reached a steady level of between 102°F. and 103°F. after about 12 hours of exposure. Superimposed on this steady level was a clearcut circadian rhythm in rectal temperature.

At 104°F. the mean heart rate continued to rise until the men were withdrawn when it had reached 148 beats per minute. In all the other air conditions, the heart rates reached a relatively steady level and the maximum level attained was proportionate to the severity of the air conditions.

The sweat rate increased steadily between 96°F. and 100°F. W.B. from 130 ml./hour to 300 ml./hour. Above 100°F. it increased sharply to reach 800 ml./hour at 102°F. and 875 ml./hour at 104°F. W.B. In contrast to the sweat rate, the hourly rate of water intake did not alter between 96°F. and 100°F. W.B., being approximately constant at 150 ml./hour, but at 102°F. and 104°F. W.B. it rose sharply to 310 ml./hour and 330 ml./hour respectively. The hourly urine output was 95 ml./hour at 96°F. W.B. but fell to 50 ml./hour at 98°F. W.B. and declined further to 35 ml./hour at 104°F. W.B.

At 96°F. and 98°F. W.B. the water intake was approximately equal to the rate of sweat loss, but at 100°F. the sweat rate increased without any rise in water intake, so that the difference was about 150 ml./hour. At 102°F. and 104°F. W.B. the rate of sweating increased sharply. Although the rate of drinking also rose, the difference between intake and output had also increased to about 550 ml./hour at 104°F. W.B.

The curve of the average rate of dehydration is very similar to that representing the rate of sweating. There was little difference in the rate of dehydration between 96°F. and 98°F. W.B., the rate being 80 ml./hour, but at 100°F. it rose to 210 ml./hour. Above 100°F. W.B. the rate of dehydration rose sharply as the rate of water intake lagged further behind the rate of sweating; it was 510 ml./hour at 104°F. W.B.

The main psychological effects were on the men's temperaments. A number of men became aggressive, a few became hysterical, and a few maintained a stoical silence. At 96°F. W.B., when the men remained in the climatic rooms for 48 hours, there were periods of aggressive behaviour which alternated with apathy. Electroencephalograms were done on some subjects. No abnormalities of any kind were evoked or caused by exposure to heat.

The tolerance times before which men, who were initially cool, would not have reached their limit of tolerance to heat were estimated.