A sodium-calcium chloride spring — a salty, heat-retaining water that shows a pale milky-white tint at the source, with a faint hydrogen sulfide note and a moderately salty taste. Neutral in pH and moderately mineralized (about 5,584 mg/kg dissolved solids), it has noticeable body and holds warmth well.
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⁺1,429mgP89 national65%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺468mgP93 national25%
- Magnesiummild; a laxative if you drink itMg²⁺110mgP92 national10%
- PotassiumK⁺12.9mgP50 national0.3%
- Iron (II)Fe²⁺1.2mgP65 national0.0%
- ManganeseMn²⁺0.20mgP45 national0.0%
- CopperCu²⁺0.01mgP71 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,066mgP90 national90%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻408mgP82 national9%
- BicarbonateHCO₃⁻42.7mgP25 national0.7%
- FluorideF⁻0.60mgP38 national0.0%
- ThiosulfateS₂O₃²⁻0.10mgP45 national0.0%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃43.6mgP24 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.7mgP16 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.