A sodium-calcium chloride spring — salty and heat-retaining. Moderately mineralized at 6,735 mg/kg of dissolved solids with a neutral pH, it carries a gray tint at the source and leaves a faint salt film on the skin. The water is chlorine-disinfected before use.
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
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⁺1,758mgP90 national65%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺606mgP93 national26%
- Magnesiummild; a laxative if you drink itMg²⁺114mgP91 national8%
- PotassiumK⁺32.2mgP76 national0.7%
- AmmoniumNH₄⁺9.6mgP94 national0.5%
- Iron (III)Fe³⁺3.1mgP91 national0.1%
- ManganeseMn²⁺1.7mgP92 national0.1%
- AluminumAl³⁺0.20mgP69 national0.0%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,920mgP90 national97%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻197mgP56 national3%
- BromideBr⁻12.9mgP88 national0.1%
- IodideI⁻5.5mgP97 national0.0%
- SulfateSO₄²⁻1.8mgP11 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Fluoridetrace; drinking limits applyF⁻—mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃58.0mgP36 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.4mgP27 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.5mgP19 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- ChromiumCr< —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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.