Comparison of low-concentration carbon dioxide-enriched and tap water immersion on body temperature after passive heating

BACKGROUND: Because carbon dioxide (CO(2))-enriched water causes cutaneous vasodilation, immersion in CO(2)-enriched water facilitates heat transfer from the body to the water or from the water to the body. Consequently, immersion in CO(2)-enriched water raises or reduces body temperature faster than immersion in fresh water. However, it takes time to dissolve CO(2) in tap water and because the dissolved CO(2) concentration decreases over time, the actual CO(2) concentration is likely lower than the stated target concentration. However, it is unclear whether water containing a lower CO(2) concentration would also cool the body faster than fresh water after body temperature had been increased. METHODS: Ten healthy males (mean age = 20 ± 1 years) participated in the study. Participants 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 CO(2)-enriched water bath at 30 °C to reduce body temperature. The CO(2) concentration was set at 500 ppm. The present study measured cooling time and cooling rate (slope of the regression line relating auditory canal temperature (T(ac)) to cooling time) to assess the cooling effect of CO(2)-enriched water immersion. RESULTS: Immersion in 40 °C tap water caused T(ac) to rise 0.64 ± 0.25 °C in the tap water session and 0.62 ± 0.27 °C in the CO(2)-enriched water session (P > 0.05). During the 30 °C water immersion, T(ac) declined to the baseline within 13 ± 6 min in tap water and 10 ± 6 min in CO(2)-enriched water (P > 0.05). Cooling rates were 0.08 ± 0.06 °C/min in tap water and 0.08 ± 0.04 °C/min in CO(2)-enriched water (P > 0.05). CONCLUSIONS: CO(2)-enriched water containing 500 ppm CO(2) did not cool faster than tap water immersion. This suggests that when the water temperature is 30 °C, a CO(2) concentration of 500 ppm is insufficient to obtain the advantageous cooling effect during water immersion after body temperature has been increased.