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 (Trec) 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 Trec 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 Trec during the final 60 min (ΔTrec (T120-T60) ≥ 0.45°C) was similar (p = 0.735) PreHA (15% HI) and PostHA (10%), and HA reduced ΔTrec (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.