Evaluation of Tolerance Limits for Humans under Heat Stress and the Problems Involved

• Published in: Scandinavian Journal of Work, Environment ＆ Health Vol. 15; no. 1; pp. 7 - 14
• Main Authors: Wenzel, Hans G, Mehnert, Claus, Schwarzenau, Paul
• Format: Journal Article
• Language: English
• Published: Finland Scandinavian Journal of Work, Environment ＆ Health 01.01.1989; National Institute of Occupational Health (Sweden); Institute of Occupational Health (Finland); National Institute of Occupational Health (Denmark); National Institute of Occupational Health (Norway)
• Subjects: Ambient temperature; Body Temperature; Climate models; Heart rate; Heat stress disorders; Hot Temperature - adverse effects; Humans; Humidity; Humidity - adverse effects; Isotherms; Male; Physical Endurance; Physical Exertion - physiology; Physiological responses; Relative humidity; Risk Factors; Stress, Physiological - physiopathology; Sweat; Weight loss; individual health risk; heat stress indices; criteria of heat tolerance
• ISSN: 0355-3140; 1795-990X

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°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°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°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°C isotherm might be taken into consideration as a limit.