酸性泉

At least 1 mg/kg of hydrogen ions; some examples sit below pH 2, strongly enough to sterilise. Kusatsu is the archetype. Used for skin conditions, but harsh enough that the local custom is to rinse with fresh water afterwards rather than let it dry on you.

What it is like

Clear to faintly milky. Sour on the tongue, sharply so. You will discover every small cut you have.

Indications (適応症)

Atopic dermatitis, psoriasis, impaired glucose tolerance, pustular skin conditions

Contraindications (禁忌症)

Too aggressive for fragile skin or mucous membranes, and for many older bathers. It stings badly in the eyes. Remove metal jewellery — it will tarnish.

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.