A sodium-calcium chloride spring — salty and heat-retaining, with noticeable body from a moderate 2,296 mg/kg of dissolved solids. It emerges hot at 45.2 °C and is weakly alkaline, soft on the skin. The water carries a sulfur odor, with a salty, faintly metallic taste at the wellhead.
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
Take care
- acute illness (especially with fever)
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⁺633mgP79 national77%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺160mgP81 national22%
- PotassiumK⁺12.2mgP49 national0.9%
- LithiumLi⁺0.60mgP61 national0.3%
- MagnesiumMg²⁺0.60mgP21 national0.1%
- StrontiumSr²⁺1.1mgP79 national0.1%
- AmmoniumNH₄⁺0.10mgP24 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- BariumBa²⁺0.30mgP77 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,225mgP82 national93%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻60.0mgP44 national3%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻61.6mgP33 national3%
- FluorideF⁻2.0mgP59 national0.3%
- BromideBr⁻3.5mgP77 national0.1%
- AsO2AsO₂⁻0.0000mg0.0%
- BO2BO₂⁻0.0000mg0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- HSiO3HSiO₃⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃107mgP57 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂28.8mgP79 national
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
- ArsenicAs0.0020mgP8 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- chronic muscle or joint pain and stiffness
- peripheral circulatory disorders
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.