Background

It is well documented that immersion in CO2-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 CO2-rich (1000 ppm) water with immersion in tap water after passive heating [8]. In that study, it was observed that immersion in CO2-rich water reduced ear canal temperature (Tac) 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 CO2-rich water than in tap water. In both of those earlier studies, the CO2 concentration was set at 1000 ppm because that concentration is the lowest found in therapeutic springs [5].

However, there are problems associated with preparing artificial CO2-enriched water. For example, it takes time to dissolve CO2 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 CO2-rich water [2, 8]. In addition, the CO2 concentration gradually decreases over time. Considering actual usage, therefore, it is likely the CO2 concentration in the CO2-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 CO2 in the CO2-enriched water immersion can cool a body faster than tap water immersion. Schnizer et al. [6] previously examined the effect of CO2-enriched water on skin blood flow at water temperatures ranging from 22 to 38 °C and CO2 concentrations ranging from 0 to 4000 ppm. They showed that the amount of increase in skin blood flow depended on the CO2 concentration and suggested that the minimal effective concentration is 400–600 ppm. However, it is unknown whether water enriched with 400–600 ppm CO2 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 CO2-enriched water, the present study compared the cooling rates between water enriched with a lower concentration of CO2 and tap water during whole body water immersion after passive heating.

Material and methods

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 CO2-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 (Tac). During the experiments, Tac 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 (\(\overline{\mathrm{T}}\)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 CO2-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 CO2-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 CO2-enriched water, the water (both tap water and CO2-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) Tac dropped to baseline level, (2) the participants asked to terminate the experiment, or (3) Tac did not change after more than 3 min. The CO2-enriched water was prepared using a device designed to dissolve CO2 in tap water (SC401, Mitsubishi Chemical Aqua Solutions, Tokyo, Japan) [2, 8]. In previous studies, where the water was enriched with 1000 ppm CO2 [2, 3, 5, 6, 8], it took > 20 min to prepare a full bathtub (about 200 L) of CO2-enriched water [2, 8]. The present study aimed to prepare 400–600 ppm CO2-enriched water within < 10 min. As a result, the CO2 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.

Fig. 1
figure 1

The experiment protocol. The participants initially sat in a chair for 5 min. During that time, baseline values were measured for each parameter. After the baseline measurements, the participants moved to a bath and were immersed in tap water at 40 °C for 15 min. The participants then moved to another bath and were immersed in tap water or CO2-enriched water at 30 °C

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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 CO2-enriched water) and condition (levels: tap and CO2-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 CO2-enriched water sessions with respect to the changes in Tac from that reached during the immersion in 40 °C tap water, the cooling times, cooling rates (slope of the regression line between Tac and cooling time), thermal comfort, and thermal sensation. Values of P < 0.05 were considered significant.

Results

At baseline, Tac was 36.0 ± 0.5 °C in the tap water condition and 35.8 ± 0.6 °C in the CO2-enriched water condition. Figure 2 shows the time-dependent changes in Tac. After immersion for 15 min in the 40 °C tap water bath, Tac had risen by 0.64 ± 0.25 °C in the tap water condition and by 0.62 ± 0.27 °C in the CO2-enriched water condition (P > 0.05) (Fig. 2A). After subsequent immersion in the 30 °C bath, the time required for Tac to return to baseline was 13 ± 6 min in the tap water condition and 10 ± 6 min in the CO2-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 CO2-enriched water condition (P > 0.05). Tac did not return to the baseline level in three participants in the tap water condition and in one participant in the CO2-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).

Fig. 2
figure 2

Time-dependent changes in change in auditory canal temperature (A) and mean skin temperature (B). The numbers adjacent to the symbols in (A) indicate the numbers of participants still immersed at the corresponding time; the numbers in (A) also apply to (B). Condition and time are the two factors considered in the ANOVA; condition × time is their interaction. BL baseline, EH end of heating

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Before heating, \(\overline{\mathrm{T}}\)sk was 31.7 ± 0.7 °C in the tap water condition and 31.6 ± 1.1 °C in the CO2-enriched water condition. After immersion for 15 min in the heated bath, \(\overline{\mathrm{T}}\)sk had risen to 38.4 ± 0.6 °C in tap water and to 38.4 ± 0.4 °C in CO2-enriched water. \(\overline{\mathrm{T}}\)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 CO2-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 CO2-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).

Fig. 3
figure 3

Time-dependent changes in heart rate. Because heart rate could not be measured in one subject, the data presented are from the remaining nine subjects. The numbers adjacent to the symbols indicate the numbers of participants still immersed at the corresponding time. Condition and time are the two factors considered in the ANOVA; condition × time is their interaction. BL baseline, EH end of heating

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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.

Table 1 Thermal comfort and sensation during cooling
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Conclusions

The present study shows that CO2-enriched water containing 500 ppm CO2 does not facilitate heat transfer from the body to the water. It was previously reported that immersion in CO2-enriched water containing 1000 ppm CO2 cooled the body 1.7 times faster than immersion in tap water [8]. That was not the case with the lower concentration of CO2 used in the present study. Schnizer et al. [6] examined the effect of CO2-enriched water on skin blood flow at different water temperatures (22–38 °C) and CO2 concentrations (0–4000 ppm). They reported that the magnitude of the increase in skin blood flow depended on the CO2 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 CO2 concentration on skin blood flow in rats while dissolving CO2 into the freshwater bath during a 20-min water immersion. They reported that skin blood flow gradually increased during the CO2-enriched water immersion and that skin blood flow increased with increases in the dissolved CO2 concentration, even at a water temperature of 23 °C. These results suggest that a CO2 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 CO2 concentration after passive heating does not alleviate the sensation of discomfort. Previous studies reported that immersion in CO2-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 ΔTac, thermal comfort, or thermal sensation, it is suggested that, at 500 ppm, there is insufficient diffusion of CO2 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 CO2 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 \(\overline{\mathrm{T}}\)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 CO2-enriched water immersion was dependent on the water temperature. It is therefore possible that the amount of CO2 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 CO2 (500 ppm) does not cool the body faster than immersion in tap water after passive heating, nor does it alleviate the sensation of discomfort when the water temperature is set at 30 °C. These observations suggest that a higher CO2 concentration is necessary to obtain the advantageous cooling effect and alleviate discomfort during water immersion after hyperthermia. For practical application, the present results suggest that a high concentration of CO2 in water is necessary to enhance body temperature reduction, even if it takes time to prepare, and that it is necessary to maintain that high CO2 concentration throughout the immersion period.