Our aim was to identify proteins that reflect an acute systemic response to prolonged hyperbaric stress and discover potential biomarker pathways for pulmonary O₂ toxicity. The study was a double-blind, randomized, crossover design in trained male Navy diver subjects. Each subject completed two dry resting hyperbaric chamber dives separated by a minimum of one week. One dive exposed the subject to 6.5 h of 100% oxygen (O₂) at 2ATA. The alternate dive exposed the subjects to an enhanced air nitrox mixture (EAN) containing 30.6% O₂ at the same depth for the same duration. Venous blood samples collected before (PRE) and after (POST) each dive were prepared and submitted to LC-MS/MS analysis (2 h runs). A total of 346 total proteins were detected and analyzed. A total of 12 proteins were significantly increased at EANPOST (vs. EANPRE), including proteins in hemostasis and immune signaling and activation. Significantly increased proteins at O₂PRE (vs. O₂POST) included neural cell adhesion molecule 1, glycoprotein Ib, catalase, hemoglobin subunit beta, fibulin-like proteins, and complement proteins. EANPOST and O₂POST differed in biomarkers related to coagulation, immune signaling and activation, and metabolism. Of particular interest is (EANPOST vs. O₂POST), which is protective against oxidative stress.

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 m ² , 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 (V̇O ₂ max, 44.5 ± 5.1 vs. 51.5 ± 5.7 mL·min ⁻¹ ·kg ⁻¹ , p < 0.001), and velocity at V̇O ₂ max (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.