A novel cooling method using carbon dioxide-rich water after passive heating

In the event of a marked rise in body temperature, it is often necessary to reduce the temperature quickly. One method to rapidly drop body temperature is cold water immersion. Because carbon dioxide (CO₂)-rich water causes cutaneous vasodilation, it may be that CO₂-rich water reduces body temperature faster than fresh water. To test that idea, I compared the effects of CO₂-rich and tap water immersion on auditory canal temperature (T_ac) after passive heating. Nine healthy male subjects participated in the study. Subjects were first immersed for 15 min in a tap water bath at 40 °C to raise body temperature. They then moved to a tap water or CO₂-rich water bath at 30 °C to reduce body temperature. Immersion in 40 °C tap water caused T_ac to rise 0.64 ± 0.11 °C in the tap water session and 0.71 ± 0.25 °C in the CO₂-rich water session (P > 0.05). During the 30 °C water immersion, T_ac declined and was significantly lower in CO₂-rich water than in tap water. Cooling rates were 0.06 ± 0.04 °C/min in tap water and 0.11 ± 0.05 °C/min in CO₂-rich water (P < 0.05). In addition, both thermal sensation (3.2 ± 1.0 vs. 2.1 ± 0.9; P < 0.01) and thermal comfort (1.2 ± 0.4 vs. 2.1 ± 0.8; P < 0.01) were significantly better in CO₂-rich water than tap water. These results suggest that CO₂-rich water immersion reduces T_ac 1.7 times faster than tap water immersion, and that CO₂-rich water immersion cools the body more comfortably than tap water immersion.