Physiological and Symptomatic Responses to Heat Exposure Above Thermal Compensability in Older Women

BACKGROUND: Thermoregulatory compensability is maintained up to a wet-bulb temperature (Tw) of ~32°C in young adults (PMID: 40163728) but is predicted to occur at lower Tw in older females (PMID: 39005081). These predictions, however, lack empirical validation, and physiological strain—reflected by increases in core temperature and heart rate above the compensability threshold—remains poorly characterized. Moreover, the tolerability of prolonged heat exposure, particularly the associated heat-illness symptoms such as nausea, dizziness, and headaches, above this limit above has not been quantified in older females. HYPOTHESIS. We hypothesized that, during a 9-hour exposure above the thermal compensability limit, heat-illness symptoms, rather than physiological strain, would determine tolerable exposure duration, with symptomatic participants more likely to terminate early (non-completers) despite experiencing similar strain to asymptomatic completers. METHODS: Eight older females (mean [SD] age: 71 [4] y) completed a seated humidity-ramp protocol and two seperate 9-hour exposures. During the ramp, relative humidity (RH) increased by 3% every 10 minutes at 42°C until 70% RH was reached. The Tw limit for thermal compensability was identified from the inflection point in core (rectal) temperature (Tcore). A subsequent exposure at 42°C was conducted with RH adjusted to achieve conditions 1.3°C Tw above each participant’s compensability limit, alongside with a control condition at 28°C and 35% RH. Tcore and heart rate were recorded continuously. Exposures were terminated if participants requested withdrawal due to intolerable heat-illness symptoms (non-completers). RESULTS: The mean compensability limit was 32.8 [1.0]°C Tw (53% RH). Four participants terminated the above-compensability exposure early (mean duration: 6.1 [1.0] hours). End-exposure Tcore was 1.5°C higher than control (95% CI:1.5, 1.6; P < 0.001), though early terminators had Tcore 0.4°C lower than completers (0.3, 0.6; P < 0.001). Tcore increased continuously above the compensability limit at 0.2°C·h - ¹ (0.1, 0.3; P = 0.0043), with no difference between groups (P = 0.89). End-exposure heart rate rose by 40 beats·min - ¹ (38, 41) and increased 3 beats·min - ¹·h - ¹ overall (1, 5; both P < 0.001), with no differences between early terminators and completers (P ≥ 0.20). CONCLUSION: Exposure above the thermal compensability limit induced progressive core temperature and cardiovascular strain in older females. Yet, half terminated early due to heat-illness symptoms despite similar physiological strain to completers, highlighting that symptom onset, rather than strain alone, may drive tolerability to prolonged uncompensable heat. FUNDING: Health Canada/ Santé Canada (The views expressed herein do not necessarily represent the views of Health Canada). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.