A sodium-and-calcium chloride spring of moderate mineralization (3,437 mg/kg dissolved solids). At the wellhead the water is 35.4 °C, faintly yellow and clear, with a salty taste and a faint sulfur-and-metallic note. Weakly alkaline pH gives a soft feel on the skin while the chloride base contributes the characteristic heat-retaining quality of salt 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
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⁺924mgP83 national70%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺281mgP87 national25%
- Potassiumminor; matters for drinking limitsK⁺65.5mgP88 national3%
- Magnesiummild; a laxative if you drink itMg²⁺13.1mgP64 national2%
- AmmoniumNH₄⁺1.1mgP67 national0.1%
- Iron (II)Fe²⁺1.1mgP63 national0.1%
- StrontiumSr²⁺1.2mgP79 national0.1%
- ManganeseMn²⁺0.60mgP68 national0.0%
- AluminumAl³⁺0.10mgP54 national0.0%
- LithiumLi⁺0.10mgP29 national0.0%
- BariumBa²⁺0.10mgP53 national0.0%
- Iron (III)Fe³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,757mgP85 national87%
- Hydrogen phosphateHPO₄²⁻315mgP99 national11%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻54.9mgP29 national2%
- BromideBr⁻5.8mgP83 national0.1%
- IodideI⁻4.2mgP92 national0.1%
- AsO2AsO₂⁻0.0000mg
- BO2BO₂⁻0.0000mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.0000mg
- Fluoridetrace; drinking limits applyF⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- HSiO3HSiO₃⁻0.0000mg
- HSO4HSO40.0000mg
- HydroxideOH⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃10.8mgP2 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.4mgP22 national
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂17.6mgP50 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.01mgP25 national
- CadmiumCd< —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.
- 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.