Validity Of A Humidity Ramp Protocol For Identifying Environmental Limits Of Human Thermoregulatory Compensability

Recent work suggests that environmental limits for human thermoregulatory compensability occur at <32 °C wet bulb temperature (Tw), considerably lower than the theorized 35 °C threshold. Newly proposed empirical limits were estimated from Tw at the point of core temperature inflection in young adults exposed to ramping air temperature (Ta) or relative humidity (RH). Despite their growing use, however, the validity of ramp protocols has not been established. PURPOSE: We assessed whether heat balance and stable core temperature (compensability) are achieved below but not above the core temperature inflection point. METHODS: Twelve adults (3 females, median (IQR) age: 28 (25-31) yrs) completed a ramp protocol in which Ta was held at 42 °C while RH was increased 3% every 10 min (28 to 70%). The critical Tw for compensability was estimated from the esophageal temperature inflection point. Participants then completed 3 randomized exposures on separate days. In two of these, participants rested in 42 °C Ta while Tw was increased to and then held at ~1.3 °C (~6% RH) above or below their predetermined critical value (TAbove and TBelow). The third was a control condition (28 °C Ta, 35% RH). Exposure continued until participants requested to stop, rectal temperature (Tre) exceeded 39.2 °C, or 9 h elapsed. RESULTS: The critical Tw was ~32.3 °C (51% RH), producing TAbove of ~33.7 °C (57% RH) and Tbelow of ~30.9 °C (45% RH). Nine participants did not complete TAbove and 1 did not complete TBelow. Peak Tre was 39.2 (39.0-39.2)°C in TAbove, 38.2 (37.9-38.4)°C in TBelow , and 37.2 (37.2-37.4)°C in control. Tre rose continuously by 0.3 °C/h [95%CI: 0.3, 0.4] and 0.1 °C/h [0.0, 0.1] in TAbove and TBelow, respectively (P ≤ 0.01), but was stable in control (0.0 °C/h [-0.1, 0.1]; P = 0.17). In TAbove and TBelow, participants also exhibited continual rises in skin temperature (0.3 °C/h [0.2, 0.4] and 0.1 °C/h [0.0, 0.2]) and heart rate (6 bpm/h [4, 8] and 4 bpm/h [2, 7]; P ≤ 0.01). CONCLUSION: Core temperature inflection clearly demarcated the ambient conditions above which heat balance cannot be achieved and severe hyperthermia rapidly develops. Below this threshold, stable body temperatures were not achieved, though rates of rise were slower than in Tabove. Our findings suggest that ramp protocols overestimate the environmental limits for compensability. Natural Sciences and Engineering Research Council of Canada and Mitacs (ClinicalTrials.Gov: NCT05601713)