Effects of repeated carbon dioxide-rich water bathing on core temperature, cutaneous blood flow and thermal sensation

We examined the effects of repeated artificial CO(2) (1,000 ppm) bathing on tympanic temperature (T(ty)), cutaneous blood flow, and thermal sensation in six healthy males. Each subject was immersed in CO(2)-rich water at a temperature of 34 degrees C up to the level of the diaphragm for 20 min. The CO(2)-rich water was prepared using a multi-layered composite hollow-fiber membrane. The CO(2) bathing was performed consecutively for 5 days. As a control study, subjects bathed in fresh water at 34 degrees C under the same conditions. T(ty) was significantly lowered during CO(2) bathing (P < 0.05). Cutaneous blood flow in the immersed skin (right forearm) was significantly increased during CO(2) bathing compared with that during fresh-water bathing (P < 0.05), whereas cutaneous blood flow in the non-immersed skin (chest) was not different between CO(2) and fresh-water bathing. Subjects reported a "warm" sensation during the CO(2) bathing, whereas they reported a "neutral" sensation during the fresh-water bathing. The effects of the repeated CO(2) bathing were not obvious for core temperature and cutaneous blood flow, but the thermal sensation score during the CO(2) bathing was reduced sequentially by repeated CO(2) bathing (P < 0.05). These thermal effects of CO(2) bathing could be ascribed largely to the direct action of CO(2) on vascular smooth muscles and to the activity of thermoreceptors in the skin. Serial CO(2) bathing may influence the activity of thermoreceptors in the skin.