Body cooling effects of immersion of the forearms in high-concentration artificial carbonic acid water at 25°C

This study examined the effects of immersion in stirred, high-concentration, artificial carbonic acid water on body cooling.

Seven healthy male students (23 ± 2 years old) participated in the experiment. Signed informed consent was obtained from all subjects before the experiment. The subjects changed into shorts and T-shirts and entered an experimental room (with room temperature controlled at 30 °C and relative humidity maintained at 70%) at least 30 min before starting the experiment. After starting the experiment, the subjects were asked to rest on an exercise bike for 5 min and then pedal for 20 min. The exercise load was set to reach 50% of each subject's presumed maximum oxygen intake at 5 min after starting exercise. Subjects then continued pedaling for 1 min to cool down. After this exercise, subjects sat on a chair and immersed forearms in tap water or artificial carbonic acid water (CO₂ water) at 25 °C for 20 min. During immersion, tap water or CO₂ water was stirred slowly with a pump. After immersion, subjects rested for 10 min. Skin temperature and skin blood flow (left forearm), as well as heart rate and ear canal temperature, were measured continuously. Thermal sensation and thermal comfort were measured intermittently.

Skin blood flow of the immersed forearms was higher in CO₂ water than in tap water during immersion. The blood flow in the last 5 min (average at rest was 100%) was significantly higher in CO₂ water (290.85 ± 84.81%) than in tap water (104.80 ± 21.99%). Thermal sensation and thermal comfort were not different between conditions. Ear canal temperature significantly declined more in CO₂ water (- 0.56 ± 0.31 °C) than in tap water (- 0.48 ± 0.30 °C) during immersion.

Our study suggests that immersion of the forearms in slowly stirred CO₂ water at 25 °C reduces core temperature elevated by heat stress or exercise more effectively than does tap water at the same temperature. Immersion of the forearms in stirred CO₂ water at 25 °C could be useful as a preventive measure against heat stroke from summer work or exercise.