A comparative study was made on bathing-induced changes in body temperature, blood pressure, pulse rate and tissue blood flow of 12 healthy adults using tap water and artificial CO₂ water at 37°C to clarify the physiological effects of CO₂ at various concentrations of 0, 100, 300, 600, 800 and 1000ppm. There was no change in body temperature during bathing in either water, whereas blood pressure and pulse rate were similarly decreased during bathing, but either of these decreases was not significantly different between tap water and CO₂-water. Therefore, it seemed that the decrease in blood pressure due to vasodilation during bathing would be controlled through some regulatory system like autonomic nerve system not so as to result in too much decrease. But, tissue blood flow was more increased during bathing in CO₂-water than tap water, suggesting that blood circulation in the tissue near skin surface would have been more enhanced by bathing in hot CO₂-water, resulting in a decrease of blood pressure.
Since the increase in tissue blood flow during bathing was dependent on the concentration of CO₂, it seemed that an improvement of tissue circulation and metabolism would have resulted from venous return increase associated to venous dilatation, one of dose-dependent CO₂ effects.