Heat tolerance classification criteria require population‐specific thresholds for accurate assessment of acclimation state in adults

Heat tolerance testing (HTT) assesses responses to heat stress with rectal temperature (T_rec) and heart rate (HR) thresholds defining individuals as heat-tolerant (HT) or heat-intolerant (HI). To evaluate classification criteria by acclimation state and sex. Forty participants (19M/21F, mean ± SD, 23 ± 4 years) completed an HTT (120 min, 5 km·h^-1, 2% grade) before (PreHA) and after (PostHA) 5 days of repeated exercise in (40°C, 40% RH) categorized as isothermal (exercise intensity adjusted to maintain T_rec within 38.5°C-39.5°C, 60 min) exercise-heat acclimation (HA). Females had lower body surface area (vs. males) (1.7 ± 0.1 vs. 2.0 ± 0.2 m², p < 0.001), mass (59.5 ± 5.4 vs. 79.5 ± 10.1 kg, p < 0.001), height (164.2 ± 5.7 vs. 179.1 ± 7.6 cm, p < 0.001), maximal oxygen consumption (V̇O₂max, 44.5 ± 5.1 vs. 51.5 ± 5.7 mL·min^-1·kg^-1, p < 0.001), and velocity at V̇O₂max (12.9 ± 1.5 vs. 15.0 ± 1.7 km·h^-1, p < 0.001). PreHA 51.1% (mean % HI classification across criteria) of participants were HI (66.7% F/52.6% M), decreasing (p < 0.001) to 23.6% PostHA (28.6% F/10.5% M), reflected across criteria (p < 0.05). The rate of HI based on plateau in T_rec during the final 60 min (ΔT_{rec (T120-T60)} ≥ 0.45°C) was similar (p = 0.735) PreHA (15% HI) and PostHA (10%), and HA reduced ΔT_{rec (T120-T60)} (0.30 ± 0.19 vs. 0.19 ± 0.20°C, p = 0.001), demonstrating sensitivity to adaptation. HA reduces HI classification rate, but criteria differ in capturing presumed HT and classify more females as HI in naïve and acclimated states.