A cold-mineral source (19.6 °C) heated for bathing, classified as a strongly alkaline simple thermal water. Lightly mineralized at just 0.326 g/kg dissolved solids, it is mild on the body, while the pH 9.5 strong alkalinity delivers the slippery, beauty-water feel on the skin that this style is known for.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
From a certificate analyzed in 2009.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were removed from the official list of contraindications then, or include a condition that was. If you have a health condition, ask a doctor before bathing.
- Acute illness
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Respiratory failure
- Renal failure
- Bleeding disorders
- Any illness in an actively progressing stage
- Pregnancy (especially the early and late stages) (dropped from the official list in 2014)
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺87.7mgP25 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺6.4mgP20 national
- Potassiumminor; matters for drinking limitsK⁺1.0mgP13 national
- StrontiumSr²⁺0.60mgP73 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻111mgP45 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻51.2mgP93 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻13.0mgP14 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻9.1mgP92 national
- Fluoridetrace; drinking limits applyF⁻1.4mgP53 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃28.8mgP11 national
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.