硫黄泉

At least 2 mg/kg of total sulfur. The hydrogen-sulfide type carries the rotten-egg smell that most people actually mean when they say something "smells like an onsen". Dilates peripheral vessels and softens the skin's keratin. Frequently a milky white.

What it is like

Usually milky white or a bluish white, often with 湯の花 — flakes of mineral precipitate — drifting in it. The smell arrives before you reach the building.

Indications (適応症)

Atopic dermatitis, psoriasis, chronic eczema, pustular skin conditions, poor peripheral circulation. Where drinking is permitted: diabetes, high cholesterol.

Contraindications (禁忌症)

Hydrogen sulfide is heavier than air and pools in hollows and unventilated rooms; this has killed people at Japanese onsen. Take posted ventilation warnings literally. Harsh on fragile skin. Silver jewellery will blacken — leave it in the locker.

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.