A hypertonic sodium-calcium-magnesium chloride spring. Richly mineralized at 14.34 g/kg total dissolved solids — the bath leaves a salty mineral film on the skin and a long-lingering warmth typical of high-salt chloride waters. Known by its legacy name as earth-chloride / common-salt-type, an 'earth-chloride / common-salt' spring in which calcium and magnesium share the cation load with sodium.
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
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⁺2,729mgP93 national46%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,600mgP97 national31%
- Magnesiummild; a laxative if you drink itMg²⁺667mgP99 national21%
- StrontiumSr²⁺—mg2%
- PotassiumK⁺37.6mgP80 national0.4%
- BariumBa²⁺25.3mgP99 national0.1%
- Iron (II)Fe²⁺10.1mgP87 national0.1%
- LithiumLi⁺0.90mgP72 national0.1%
- ManganeseMn²⁺1.6mgP88 national0.0%
- AmmoniumNH₄⁺0.50mgP49 national0.0%
- AluminumAl³⁺0.10mgP54 national0.0%
- ZincZn²⁺0.03mgP57 national
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8,867mgP96 national99%
- BicarbonateHCO₃⁻111mgP44 national0.7%
- SulfateSO₄²⁻20.6mgP32 national0.2%
- IodideI⁻3.9mgP92 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃24.1mgP9 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.4mgP14 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂39.6mgP64 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0010mgP6 national
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.