Suitability of Thermal Indices in Describing Heat Stress Compensability.

Multiple thermal indices have been created to categorize the impact of the ambient environment on human physiology and health. Many of these indices promulgate thresholds that are used to delineate between safe and unsafe thermal environments at rest and during physical activity (exercise, sport, industry, etc.). While some of these thresholds were empirically derived, others were established based on models or theoretical construct with no objective physiological measurements or rationale. In a world where heat is already the leading weather-related cause of death and climate change will exacerbate the effects of extreme heat in the future, it is important to determine if commonly used indices and associated thresholds are able to accurately describe ambient conditions which are thermally safe or unsafe. One physiologically- and biophysically-determined definition of safe environments is based on the upper limit of thermal balance (PSU H.E.A.T. Project). In the present study, young, healthy adults were exposed to a progressive heat stress protocol in an environmental chamber in both hot-dry and warm-humid conditions to determine critical environmental limits of dry-bulb temperature and vapor pressure which could then be compared to other widely used critical heat stress index values. That is, upper environmental limits for human heat balance were compared to common heat stress indices, including wet-bulb temperature (Twb), Heat Index (HI), Wet Bulb Globe Temperature (WBGT), and Universal Thermal Comfort Index (UTCI). Under activities of daily living, HI and WBGT thresholds very nearly match up with heat stress compensability curves based on critical heat balance limits while large variation was found in with UTCI. Interestingly, the theorized Twbthreshold of 35°C, used increasingly by mainstream media sources in describing extreme heat, was significantly higher than the critical limit for heat stress compensability. This work highlights the need for continued empirical work in determining and validating existing critical heat stress index thresholds as well as the need for better communication to the general public of what each heat stress index means for their environmental health and safety in a warming world.