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

## TL;DR

This study shows that heat tolerance classification criteria need to consider population-specific thresholds, as they can misclassify individuals, especially females, before and after heat acclimation.

## Contribution

The study demonstrates that classification criteria for heat tolerance differ in sensitivity and may misclassify more females as heat-intolerant.

## Key findings

- Heat acclimation reduces the rate of heat-intolerant classification across criteria.
- Females are more frequently classified as heat-intolerant compared to males in both naïve and acclimated states.
- Changes in rectal temperature during the final 60 minutes of the test are sensitive indicators of acclimation.

## Abstract

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.

Heat acclimation status is detected by changes in response to a standard Israeli Defense Force (IDF) heat tolerance test (HTT) intended to evaluate safety among participants returning to duty after exertional heat illness or anticipating exposure to exercise‐heat stress. Measures of heart rate and core temperature during HTT suggest that varying classification criteria differentially designate heat tolerance status among individuals and by sex.