A sodium-led chloride spring softened by a bicarbonate sub-type — salty, heat-retaining water with a smoother edge than a pure chloride spring. Weakly alkaline at pH 8.0 and soft on the skin, it sits in the Japanese "beauty-water" family. Moderately mineralized at 2,613 mg/kg, with a generous 107 mg/kg of metasilicic acid that leaves the kind of thin silica film which makes skin feel noticeably smoother afterwards.
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
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⁺783mgP82 national93%
- Potassiumminor; matters for drinking limitsK⁺34.3mgP78 national2%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.2mgP34 national2%
- AmmoniumNH₄⁺9.7mgP95 national1%
- MagnesiumMg²⁺4.4mgP45 national1.0%
- AluminumAl³⁺0.70mgP76 national0.2%
- Iron (II)Fe²⁺1.6mgP68 national0.2%
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻890mgP80 national68%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻727mgP84 national32%
- CarbonateCO₃²⁻2.7mgP27 national0.2%
- BromideBr⁻3.5mgP77 national0.1%
- IodideI⁻1.0mgP79 national0.0%
- SulfateSO₄²⁻0.70mgP6 national0.0%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.0%
- FluorideF⁻0.10mgP9 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃107mgP57 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.0mgP71 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂14.1mgP46 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —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.
- 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.