二酸化炭素泉

At least 1,000 mg/kg of free CO₂ — genuinely rare in Japan. The gas absorbs through the skin and dilates capillaries, so even a tepid bath warms you thoroughly and blood pressure tends to fall. Because heat drives the CO₂ off, these are almost always served cool, which surprises first-timers.

What it is like

Fine bubbles cling to every submerged inch of you within a minute — the most immediately recognisable water of the ten. Prickles pleasantly, and feels warmer than the thermometer says.

Indications (適応症)

Cuts, poor peripheral circulation, sensitivity to cold, autonomic instability

Contraindications (禁忌症)

In a poorly ventilated bathhouse CO₂ can pool dangerously. Those with low blood pressure should keep soaks short.

適応症 and 禁忌症 are the claims a certificate is permitted to state under the Hot Spring Act. They are not medical advice, and nothing here is. If you have a heart condition, are pregnant, or are unwell, ask a doctor rather than a guidebook.