Core temperature responses to compensable versus uncompensable heat stress in young adults (PSU HEAT Project)

Abstract

With global warming, much attention has been paid to the upper limits of human adaptability. However, the time to reach a generally-accepted core temperature criterion (40.2°C) associated with heat-related illness above (uncompensable heat stress) and just below (compensable heat stress) the upper limits for heat balance remains unclear. Forty-eight (22 men/26 women; 23±4 y) subjects were exposed to progressive heat stress in an environmental chamber during minimal activity (MinAct, 159±34W) and light ambulation (LightAmb, 260±55W) in warm-humid (WH; ~35°C, >60% RH) and hot-dry (HD; 43-48°C, <25% RH) environments until heat stress became uncompensable. For each condition, we compared heat storage (S) and the change in gastrointestinal temperature (∆Tgi) over time during compensable and uncompensable heat stress. Using the slopes of the Tgi response, we estimated the time to reach Tgi=40.2°C. Finally, we examined whether individual characteristics or seasonality were associated with the rate of increase in Tgi. During compensable heat stress, S was higher in HD than in WH environments (p<0.05) resulting in a greater but more variable ∆Tgi (p≥0.06) for both metabolic rates. There were no differences among conditions during uncompensable heat stress (all p>0.05). There was no influence of sex, aerobic fitness, or seasonality, but a larger body size was associated with a greater ∆Tgi during LightAmb in WH (p=0.003). Sustained light activity without intervention in uncompensable thermal environments may result in a Tc of 40.2°C (from a 37°C baseline) in 3-7 hours even in young adults vs. several days under compensable heat stress.

Observational and model evidence together support wide-spread exposure to noncompensable heat under continued global warming

“…Death occurs given core temperatures greater than 43°C ( 1 ). As measured in-laboratory, when exposed to noncompensable heat, core temperature rises at an average of about 1°C/hour ( 16 ). Given at least 6 hours of exposure to noncompensable heat, therefore, a healthy, nonheat-adapted human being could see their core temperature rise to lethal levels.…”

“…Noncompensable heat stress is defined as the set of environmental conditions under which a healthy human being can no longer maintain a stable core temperature without the assistance of external cooling. All else held equal, exposure to 6 hours of noncompensable heat could result in a lethal rise in core temperatures for a healthy human being (see Materials and Methods for more details) ( 16 ).…”

A Novel Conceptual Model for Human Heat Tolerance

“…Heart rate begins to increase slowly as the CEL is approached (Cottle 2023). In environments beyond the CEL, T c increases approximately 0.5-1.5°C per hour (25). As previously described, the CEL is determined as the environmental conditions immediately before the T c inflection point during a progressive heat stress protocol.…”

“…Data at the CEL are important indicators of the transition from compensable to uncompensable heat stress at specified metabolic rates and clothing ensembles, as demonstrated by the PSU Heat project described hereafter (25,29). For the purposes of operationalizing a heat tolerance as a single metric or index, we start by recognizing that heat stress is a combination of environment and metabolic rate and, thus, needs a single metric to represent both.…”

“…Identification of CEL values at metabolic rates associated with activities of daily living provides important information regarding heat tolerance limits for free-living individuals during heat waves. Recently, the PSU HEAT project has demonstrated the CEL for young adults (18-34 y) at two low rates of metabolic heat production across a wide range of environments (25,29). Subjects were tested in eight distinct environments ranging from warm-humid (P crit trials) to hot-dry (T crit trials) during light ambulation on a treadmill (LightAmb; M net = ~130 W•m −2 ) and during minimal activity performed on a cycle ergometer (MinAct; M net = ~80 W•m −2 ).…”