A gentle simple thermal water — weakly alkaline, hypotonic, and hot at 58 °C at the wellhead. Total dissolved solids are light (828 mg/kg), so the water is soft and mineral-quiet rather than assertive. What stands out is the metasilicic acid: at 204 mg/kg it leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards, placing this firmly in the Japanese 'beauty-water' family.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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
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⁺188mgP46 national90%
- Potassiumminor; matters for drinking limitsK⁺28.2mgP72 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.0mgP12 national2%
- LithiumLi⁺0.20mgP39 national0.3%
- AmmoniumNH₄⁺0.50mgP46 national0.3%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻360mgP67 national83%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻35.0mgP25 national14%
- Fluoridetrace; drinking limits applyF⁻3.7mgP71 national3%
- SulfateSO₄²⁻2.0mgP11 national0.6%
- Hydrogen phosphateHPO₄²⁻0.90mgP78 national0.3%
- IodideI⁻0.60mgP73 national0.1%
- BromideBr⁻0.20mgP31 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃204mgP88 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.5mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.5mgP24 national
- ArsenicAs—mg
- CadmiumCd—mg
- CopperCu—mg
- Mercurytoxic trace — part of the drinking-safety checkHg—mg
- LeadPb—mg
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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.