炭酸水素塩泉

Bicarbonate is the dominant anion. Sodium-dominant examples — the old name 重曹泉 — emulsify skin oils and rinse them away, which is why they are marketed as 美人の湯 ("beautifying water"); the effect is real, if modest. Calcium- and magnesium-dominant examples are more sedative than cleansing.

What it is like

Usually clear. The give-away is the feel: genuinely slippery, the water seeming to glide rather than wet. Slightly salty-bitter if you taste it.

Indications (適応症)

Cuts, poor peripheral circulation, sensitivity to cold, dry skin. Where drinking is permitted: sluggish digestion, diabetes, high uric acid.

Contraindications (禁忌症)

Because it strips oils, a long soak can leave already-dry skin drier.

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.