A sodium chloride spring — salty, heat-retaining water at 56.5 °C from the wellhead, with the faint rotten-egg whisper of hydrogen sulfide on the nose. Richly mineralized at 13.08 g/kg of dissolved solids, it leaves a clear mineral film on the skin. The cert also reports a generous 102.5 mg/kg of metasilicic acid, putting it in the Japanese 'beauty-water' family with the kind of silica film that 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⁺3,909mgP96 national82%
- Magnesiummild; a laxative if you drink itMg²⁺283mgP98 national11%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺198mgP85 national5%
- Potassiumminor; matters for drinking limitsK⁺169mgP96 national2%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,789mgP94 national78%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,942mgP95 national15%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻628mgP90 national6%
- FluorideF⁻4.7mgP77 national0.1%
- ThiosulfateS₂O₃²⁻2.0mgP84 national0.0%
- Hydrogen sulfideHS⁻0.10mgP28 national0.0%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃103mgP55 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂53.3mgP90 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —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.