Effectiveness of high-concentration CO₂ hot water immersion in conditioning for athletes

Water immersion is a popular conditioning method; specifically, high-concentration CO₂ hot water immersion (CO₂-HWI) has become more common in recent years. This study aimed to investigate the effectiveness of CO₂-HWI in conditioning for athletes from both physiological and biochemical perspectives. Ten male university baseball players participated in this randomized crossover trial. Each participant completed three 15-min interventions after team training sessions on separate experimental days: CO₂-HWI (CO₂; water temperature, 40 °C; CO₂ concentration, 1000 ppm), tap HWI (TAP; water temperature, 40 °C), and non-water immersion (NON; room temperature, 25 °C; relative humidity 60%). Measurements were taken before and after the intervention, and change scores were calculated. Differences in change scores among the three conditions were analyzed using the Friedman test. From a physiological perspective, systolic blood pressure showed a significant decrease in the CO₂ vs. NON comparison (p < 0.05), whereas most measures (e.g., core and skin temperatures) showed similar changes in the CO₂ and TAP conditions compared with the NON condition (p < 0.05). From a biochemical perspective, salivary cortisol and secretory immunoglobulin A levels did not show significant changes. Meanwhile, salivary human herpesvirus (HHV-) 7 DNA levels showed a significant decrease in the CO₂ vs. NON comparison (p < 0.05), while changes in HHV-6 DNA levels were not statistically significant. In summary, CO₂-HWI demonstrated comparable effectiveness to TAP-HWI from both physiological and biochemical perspectives. Among these findings, CO₂-HWI may contribute to reductions in systolic blood pressure and salivary HHV-7 DNA levels.