A hot (58.3 °C) sodium chloride spring, weakly alkaline and soft on the skin. Moderately mineralized at 2,191 mg/kg dissolved solids, it carries the salty, heat-retaining body of a Na-Cl water with a bicarbonate softness layered in. The standout is its metasilicic acid content — a generous 202 mg/kg leaves the kind of silica film that places this firmly in Japan's beauty-water family.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺717mgP81 national96%
- Potassiumminor; matters for drinking limitsK⁺35.8mgP79 national3%
- CalciumCa²⁺3.6mgP14 national0.6%
- AmmoniumNH₄⁺1.4mgP71 national0.3%
- MagnesiumMg²⁺0.20mgP14 national0.1%
- LithiumLi⁺0.10mgP29 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻759mgP78 national71%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻196mgP65 national14%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻245mgP61 national13%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻16.8mgP61 national2%
- FluorideF⁻1.1mgP49 national0.2%
- BromideBr⁻2.5mgP74 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃202mgP86 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.9mgP55 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.3mgP24 national
- ArsenicAs0.01mgP25 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn< —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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.