A valid method for comparing rational and empirical heat stress indices.

No single heat stress index has gained universal acceptance within the past 20 years, despite extensive research. It is currently difficult to directly and quantitatively compare the many rational and empirical indices that are available, which results in confusion and a reluctance to change to a different index. A method is developed using the concept of limiting metabolic rate, which allows virtually all heat stress indices to be compared with one another. Because all occupational heat stress indices are based, explicitly or implicitly, on the human heat balance equation, a unique value of metabolic rate can be found that just allows an unrestricted work/rest cycle in particular environmental conditions. A comparison using this methodology shows that there are very large differences between the recommended limits under the various indices, even for similar populations of acclimatized workers.