A hot (48.4 °C) sodium-calcium chloride spring, weakly alkaline and soft on the lips with a faint salty edge. Colorless and clear at the wellhead, the lab sample picks up a faint hydrogen sulfide note about seven hours after collection. Moderately mineralized at 3,750 mg/kg dissolved solids — noticeable body and the heat-retaining feel typical of chloride waters.
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⁺759mgP82 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺569mgP93 national46%
- StrontiumSr²⁺23.0mgP97 national0.8%
- PotassiumK⁺13.6mgP52 national0.6%
- MagnesiumMg²⁺0.40mgP18 national0.1%
- AluminumAl³⁺0.0000mg0.0%
- BariumBa²⁺0.0000mg0.0%
- FeTotalFe0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,138mgP87 national94%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻173mgP62 national6%
- BicarbonateHCO₃⁻26.0mgP16 national0.7%
- FluorideF⁻1.0mgP47 national0.1%
- CarbonateCO₃²⁻0.10mgP11 national0.0%
- AsO2AsO₂⁻0.0000mg0.0%
- BO2BO₂⁻0.0000mg0.0%
- BromideBr⁻0.0000mg0.0%
- H2PO4H₂PO₄⁻0.0000mg0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- HSiO3HSiO₃⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- NitrateNO₃⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- SS²⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.7mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.0mgP24 national
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.2mgP19 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.02mgP33 national
- 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.
- 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.