Evaluation of tolerance limits for humans under heat stress and the problems involved

Investigators are divided about the limits of tolerable ambient temperatures and air humidities, even under equal boundary conditions. In 266 climatic chamber experiments, mostly lasting 4 h, 13 young healthy men were exposed to combinations of ambient temperature (20 to 55 degrees C) and relative humidity (5 to 97%) at an air velocity of 0.3 m.s-1. They were lightly clad and performed equal treadmill work (metabolic rate about 850 kJ.h-1). Mean rectal isotherms and their standard deviations were calculated from the final rectal temperatures of the 13 men and plotted on a psychrometric chart. The slopes of the isotherms were compared with those of various "indices of heat stress." The probability of reaching the potentially critical rectal temperature of 39 degrees C was calculated according to data of Wyndham & Heyns. It increased from about 1:500,000 to 1:150 in climates corresponding to mean isotherms of 37.6 and 38.2 degrees C, respectively. Most of the limits thus far proposed are situated between these two isotherms, and they differ merely in regard to health risk. It is useless to discuss "correct" or "incorrect" limits until society has decided what risk is to be accepted. An upper tolerable limit per se does not and cannot exist. For several reasons the 37.6 degrees C isotherm might be taken into consideration as a limit.