Critical Environmental Limit Protocol: Validity and Reliability (PSU HEAT)

Abstract

With increasing average global temperatures and greater frequency, duration, and severity of heat waves, there is an urgent need to better understand the specific environmental temperatures that place humans at greater risk of heat‐related morbidity and mortality. An incremental heat stress protocol can be used to establish critical environmental limits for human heat exchange, i.e., those combinations of ambient temperature and humidity above which heat stress becomes uncompensable and, therefore, unsafe. However, no studies have rigorously investigated the reliability and validity of this experimental paradigm.PURPOSEHere, we assessed the (1) between‐visit repeatability and (2) validity of an experimental paradigm that either holds ambient vapor pressure (Pa) constant and incrementally increases dry‐bulb temperature (Tdb) or vice versa.METHODSTwelve subjects (5M/7W; 25±4 yr) completed a progressive heat stress protocol during which they walked on a treadmill (2.2 mph, 3% gradient) in a controllable environmental chamber while gastrointestinal temperature (Tgi) was continuously monitored. After an equilibration period, the progressive heat stress protocol involved increasing Tdb every 5 min while Pa was held constant (Tcritexperiments), or increasing Pa every 5 min while Tdb was held constant (Pcritexperiments), until an upward inflection and continuous rise in Tgi was observed. The Tcrit or Pcrit was identified as the average Tdb or Pa, respectively, across the 2 min preceding the Tgi inflection point. For repeatability experiments, 11 subjects returned to the lab to repeat the same protocol as their first visit. For validity experiments, 10 subjects returned to the lab for a progressive heat stress trial in which Tdb or Pa was held constant at the Tcrit or Pcrit value from their first visit.RESULTSThe between‐visit repeatability for critical environmental limits was excellent (ICC = 0.98). Similarly, there was excellent agreement between original and validity trials for Tdb (ICC = 0.95) and Pa (ICC = 0.96) at the Tgi inflection point. Furthermore, the wet‐bulb temperature at the Tgiinflection point was not different during reliability (p = 0.78) or validity (p = 0.32) trials compared to original trials.CONCLUSIONTogether, these data provide strong evidence that this experimental paradigm is rigorous, reliable, and valid, supporting its use to investigate critical environmental limits to the maintenance of heat balance.