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

Background Because carbon dioxide (CO2)-enriched water causes cutaneous vasodilation, immersion in CO2-enriched water facilitates heat transfer from the body to the water or from the water to the body. Consequently, immersion in CO2-enriched water raises or reduces body temperature faster than immersion in fresh water. However, it takes time to dissolve CO2 in tap water and because the dissolved CO2 concentration decreases over time, the actual CO2 concentration is likely lower than the stated target concentration. However, it is unclear whether water containing a lower CO2 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 CO2-enriched water bath at 30 °C to reduce body temperature. The CO2 concentration was set at 500 ppm. The present study measured cooling time and cooling rate (slope of the regression line relating auditory canal temperature (Tac) to cooling time) to assess the cooling effect of CO2-enriched water immersion. Results Immersion in 40 °C tap water caused Tac to rise 0.64 ± 0.25 °C in the tap water session and 0.62 ± 0.27 °C in the CO2-enriched water session (P > 0.05). During the 30 °C water immersion, Tac declined to the baseline within 13 ± 6 min in tap water and 10 ± 6 min in CO2-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 CO2-enriched water (P > 0.05). Conclusions CO2-enriched water containing 500 ppm CO2 did not cool faster than tap water immersion. This suggests that when the water temperature is 30 °C, a CO2 concentration of 500 ppm is insufficient to obtain the advantageous cooling effect during water immersion after body temperature has been increased.


Background
It is well documented that immersion in CO 2 -enriched water causes cutaneous vasodilation at the immersed body surface [1][2][3][4][5][6][7]. This facilitates heat transfer from the body to the water, or from the water to the body. Applying this phenomenon, an earlier study compared the cooling effect of whole-body immersion in CO 2 -rich (1000 ppm) water with immersion in tap water after passive heating [8]. In that study, it was observed that immersion in CO 2 -rich water reduced ear canal temperature (T ac ) about 1.7 times faster than tap water immersion. Similarly, Tanaka et al. [7] evaluated the cooling effect of immersing the forearms in cool water and reported that the decrease in ear canal temperature was slightly greater in CO 2 -rich water than in tap water. In both of those earlier studies, the CO 2 concentration was Page 2 of 6 Hayashi Journal of Physiological Anthropology (2021) 40:20 set at 1000 ppm because that concentration is the lowest found in therapeutic springs [5]. However, there are problems associated with preparing artificial CO 2 -enriched water. For example, it takes time to dissolve CO 2 in tap water. With the device used in previous studies, it takes > 20 min to prepare a full bathtub (about 200 L) of 1000 ppm CO 2 -rich water [2,8]. In addition, the CO 2 concentration gradually decreases over time. Considering actual usage, therefore, it is likely the CO 2 concentration in the CO 2 -enriched water was lower than 1000 ppm, at least part of the time. For that reason, it is important to clarify whether a lower concentration (< 1000 ppm) of CO 2 in the CO 2 -enriched water immersion can cool a body faster than tap water immersion. Schnizer et al. [6] previously examined the effect of CO 2 -enriched water on skin blood flow at water temperatures ranging from 22 to 38 °C and CO 2 concentrations ranging from 0 to 4000 ppm. They showed that the amount of increase in skin blood flow depended on the CO 2 concentration and suggested that the minimal effective concentration is 400-600 ppm. However, it is unknown whether water enriched with 400-600 ppm CO 2 can actually cool the body faster than tap water after the body temperature has been increased. Therefore, to better understand the concentration dependency of the cooling effect of CO 2 -enriched water, the present study compared the cooling rates between water enriched with a lower concentration of CO 2 and tap water during whole body water immersion after passive heating.

Participants
Ten healthy males (mean age = 20 ± 1 (SD) years; height = 168.0 ± 4.7 cm; weight = 61.4 ± 7.9 kg) participated in the study. The participants were all nonsmokers, and none were taking any medication. The study was approved by the research ethics committee of the University of Shizuoka (#1-24) and conformed to the provisions of the Declaration of Helsinki. Written informed consent was obtained from all participants.

Experimental design
Each participant completed two sessions (immersion in tap water and in CO 2 -enriched water) within a 2-week period in random order. The participants were all asked to abstain from strenuous exercise and from consumption of alcohol during the 24 h before the experiment. In addition, all participants ate the same meal the night before the experiment and for breakfast on the day of the experiment. The experiment was conducted in the morning. After each participant came to the laboratory, they voided urine, were weighed, put on swimwear, and sat in a chair to rest. During this period, a heart rate (HR) monitor and thermocouples for recording skin temperature were attached. Then, an infrared temperature sensor was inserted into the auditory canal to record the temperature (T ac ). During the experiments, T ac data was collected using an infrared temperature sensor (BL100, Techno Next, Chiba, Japan), which was sampled every 1 s and averaged over 30-s periods. Skin temperature data were collected using copper constantan thermocouples, which were sampled every 1 s using a data logger system (DL350, Yokogawa, Tokyo, Japan) and averaged over 30-s periods. Skin temperatures were collected at four sites (chest, upper arm, thigh, and calf ) and used to calculate the weighted mean skin temperature ( T sk ) [9]. HR was recorded every 5 s using a HR monitor (S810i, Polar, Finland) and averaged over 30-s periods. Figure 1 shows the experimental protocol. While the participants continued to sit in the chair, baseline values for each parameter were measured for 5 min. Once the baseline measurements were complete, the participants moved to a bath and were immersed to the axilla in tap water at 40 °C for 15 min to raise their body temperature. The participants then moved to another bath and were immersed to the axilla in tap water or CO 2 -enriched water at 30 °C to reduce body temperature. A water temperature of 30 °C was selected to avoid causing shivering or discomfort. Thermal comfort and thermal sensation were recorded every 5 min while the participant was immersed in the 30 °C tap or CO 2 -enriched water. Thermal comfort was evaluated using a four-point scale (1: comfortable-4: very uncomfortable), while thermal sensation was evaluated using a seven-point scale (1: cold-7: hot) [10]. Because bubbles attach to the body during immersion in CO 2 -enriched water, the water (both tap water and CO 2 -enriched water) was stirred at 13 L/min to keep the participants unaware of which water they were immersed in. The experiments terminated when any of the following were occurred: (1) T ac dropped to baseline level, (2) the participants asked to terminate the experiment, or (3) T ac did not change after more than 3 min. The CO 2 -enriched water was prepared using a device designed to dissolve CO 2 in tap water (SC401, Mitsubishi Chemical Aqua Solutions, Tokyo, Japan) [2,8]. In previous studies, where the water was enriched with 1000 ppm CO 2 [2,3,5,6,8], it took > 20 min to prepare a full bathtub (about 200 L) of CO 2 -enriched water [2,8]. The present study aimed to prepare 400-600 ppm CO 2 -enriched water within < 10 min. As a result, the CO 2 concentration was maintained at about 500 ppm throughout the experiment. The experiments were carried out in a laboratory maintained at 23-25 °C and 40-60% relative humidity.

Statistical analysis
All values are reported as means ± SD. Statistical analyses were performed using IBM SPSS Statistics (version 27, IBM Corp., NY, USA). Two-way ANOVA with repeated measures was conducted using time (levels: 1, 2, 3, 4, 5, 6, 7, 8, and 9 min during immersion in 30 °C tap or CO 2 -enriched water) and condition (levels: tap and CO 2 -enriched water) as factors. Times at which the numbers of participants were reduced (≥ 10 min) were not analyzed. Paired t tests were used to compare the tap water and CO 2 -enriched water sessions with respect to the changes in T ac from that reached during the immersion in 40 °C tap water, the cooling times, cooling rates (slope of the regression line between T ac and cooling time), thermal comfort, and thermal sensation. Values of P < 0.05 were considered significant.

Results
At baseline, T ac was 36.0 ± 0.5 °C in the tap water condition and 35.8 ± 0.6 °C in the CO 2 -enriched water condition. Figure 2 shows the time-dependent changes in T ac . After immersion for 15 min in the 40 °C tap water bath, T ac had risen by 0.64 ± 0.25 °C in the tap water condition and by 0.62 ± 0.27 °C in the CO 2 -enriched water condition (P > 0.05) ( Fig. 2A). After subsequent immersion in the 30 °C bath, the time required for T ac to return to baseline was 13 ± 6 min in the tap water condition and 10 ± 6 min in the CO 2 -enriched water condition (P > 0.05). The cooling rates were 0.08 ± 0.06 °C/min in the tap water condition and 0.08 ± 0.04 °C/ min in the CO 2 -enriched water condition (P > 0.05). T ac did not return to the baseline level in three participants in the tap water condition and in one participant in the CO 2 -enriched water condition. During the cooling, there was a significant main effect of cooling time (F = 6.37, P < 0.01). However, there was no significant main effect of condition (F = 1.41, P = 0.24) and no interaction between the condition and cooling time (F = 0.23, P = 0.98). Before heating, T sk was 31.7 ± 0.7 °C in the tap water condition and 31.6 ± 1.1 °C in the CO 2 -enriched water condition. After immersion for 15 min in the heated bath, T sk had risen to 38.4 ± 0.6 °C in tap water and to 38.4 ± 0.4 °C in CO 2 -enriched water. T sk rapidly declined during cooling, and there was a significant main effect of cooling time (F = 7.13, P < 0.01), but there was no significant main effect of condition (F = 2.88, P = 0.09) and no interaction between the condition and cooling time (F = 0.11, P = 0.99) (Fig. 2B). Figure 3 shows the time-dependent changes in HR. Before heating, HR was 75 ± 6 beats/min in the tap water condition and 75 ± 5 beats/min in the CO 2 -enriched water condition. By the end of heating, HR had increased to 96 ± 6 beats/min in the tap water condition and to 95 ± 4 beats/min in the CO 2 -enriched water condition. HR decreased during cooling in both conditions, and there was a significant main effect of cooling time (F = 7.29, P < 0.01). On the other hand, there was no significant main effect of condition (F = 2.07, P = 0.15) or interaction between condition and cooling time (F = 0.10, P = 0.99). Table 1 shows the thermal comfort and sensation reported during cooling. There was no significant between-condition difference in thermal comfort or thermal sensation after 5 min of cooling.

Conclusions
The present study shows that CO 2 -enriched water containing 500 ppm CO 2 does not facilitate heat transfer from the body to the water. It was previously reported that immersion in CO 2 -enriched water containing 1000 ppm CO 2 cooled the body 1.7 times faster than immersion in tap water [8]. That was not the case with the lower concentration of CO 2 used in the present study. Schnizer et al. [6] examined the effect of CO 2 -enriched water on skin blood flow at different water temperatures (22-38 °C) and CO 2 concentrations (0-4000 ppm). They reported that the magnitude of the increase in skin blood flow depended on the CO 2 concentration and the water temperature and that the latency of the increase in skin blood flow increased with decreases in water temperature. In addition, Ito et al. [1] examined the effect of water temperature and CO 2 concentration on skin blood flow in rats while dissolving CO 2 into the freshwater bath during a 20-min water immersion. They reported that skin blood flow gradually increased during the CO 2 -enriched water immersion and that skin blood flow increased with increases in the dissolved CO 2 concentration, even at a water temperature of 23 °C. These results suggest that a CO 2 concentration of 500 ppm in water was insufficient to facilitate heat transfer from the body during a 10-min water immersion. Moreover, there was no significant between-condition difference in thermal comfort or thermal sensation, indicating that immersion in water containing a low CO 2 concentration after passive heating does not alleviate the sensation of discomfort. Previous studies reported that immersion in CO 2 -enriched water produced a warmer, more comfortable sensation than immersion in fresh water [2,5,8]. Given the present observations that there were no significant differences in ΔT ac , thermal comfort, or thermal sensation, it is suggested that, at 500 ppm, there is insufficient diffusion of CO 2 into cutaneous blood vessels to exert a beneficial effect.
On the other hand, Sato et al. [4] reported that when participants were immersed in a hot bath, even 100 ppm CO 2 enhanced skin blood flow and sweating measured at the chest as compared to freshwater immersion. The difference between the present study and Sato's study is water temperature. In their study, measurements of skin blood flow were made at a water temperature of 40 °C. During immersion in hot water, even fresh water, skin blood flow is augmented [4]. Although skin blood flow was not measured in the present study, the water temperature was set at 30 °C, and T sk was always lower than 34 °C during immersion. This suggests that the degree of cutaneous vasodilation was almost certainly smaller than in Sato's study. Previous studies [1,6] reported that the magnitude of the increase in skin blood flow during CO 2 -enriched water immersion was dependent on the water temperature. It is therefore possible that the amount of CO 2 diffusing into the cutaneous blood vessels was higher in those earlier studies than in the present study.
In summary, the results of the present study suggest that immersion in water containing a low concentration of CO 2 (500 ppm) does not cool the body faster than