A sodium-calcium chloride spring with a salty, faintly bitter character. Moderately mineralized and isotonic, it has the body and heat-retaining quality typical of chloride waters, while a mildly alkaline pH keeps it soft on the skin.
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⁺2,143mgP92 national57%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,201mgP96 national39%
- Magnesiummild; a laxative if you drink itMg²⁺4.9mgP50 national3%
- PotassiumK⁺15.9mgP55 national0.2%
- StrontiumSr²⁺14.3mgP95 national0.2%
- ManganeseMn²⁺4.5mgP97 national0.1%
- LithiumLi⁺0.30mgP44 national0.0%
- AmmoniumNH₄⁺0.80mgP61 national0.0%
- BariumBa²⁺0.40mgP81 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,884mgP94 national89%
- SulfateSO₄²⁻61.3mgP45 national0.7%
- BromideBr⁻8.5mgP88 national0.1%
- FluorideF⁻1.1mgP49 national0.0%
- IodideI⁻4.7mgP93 national0.0%
- BicarbonateHCO₃⁻1.2mgP2 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSO4HSO4< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.9mgP18 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂—mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂51.7mgP69 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.