A sodium chloride spring drawn from one of Japan's hottest natural sources — the water leaves the ground above 71 °C. It runs weakly acidic, with a faint sour note if tasted, and its 3,306 mg/kg of dissolved solids give it the heat-retaining quality chloride waters are known for: the warmth lingers well after you climb out. What stands out chemically is metasilicic acid at 357.6 mg/kg, an unusually generous figure that leaves a thin silica film on the skin and places this water squarely in Japan's beauty-water family.
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⁺931mgP83 national87%
- Potassiumminor; matters for drinking limitsK⁺114mgP93 national6%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.8mgP51 national4%
- LithiumLi⁺4.0mgP94 national1%
- MagnesiumMg²⁺3.9mgP43 national0.7%
- ManganeseMn²⁺0.70mgP72 national0.1%
- Iron (II)Fe²⁺0.60mgP56 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,397mgP83 national82%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻397mgP82 national17%
- BicarbonateHCO₃⁻7.0mgP4 national0.3%
- FluorideF⁻2.0mgP59 national0.2%
- BromideBr⁻2.4mgP72 national0.1%
- HSO4HSO40.20mgP39 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃358mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂48.6mgP87 national
- Metaarsenious acidHAsO₂1.3mgP91 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂16.9mgP49 national
- ArsenicAs0.88mgP93 national
- 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.