A hot (55.9 °C) sodium chloride-bicarbonate spring sitting at neutral pH. Mineralization is solid at 7,883 mg/kg, giving the water noticeable body and leaving a faint salt film on the skin. A generous 110.9 mg/kg of metasilicic acid layers in a thin silica film that smooths the skin 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⁺2,297mgP92 national87%
- Magnesiummild; a laxative if you drink itMg²⁺90.0mgP91 national6%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺75.8mgP66 national3%
- Potassiumminor; matters for drinking limitsK⁺133mgP94 national3%
- ManganeseMn²⁺0.70mgP72 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,746mgP89 national66%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,163mgP96 national30%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻206mgP66 national4%
- FluorideF⁻4.4mgP75 national0.2%
- CarbonateCO₃²⁻1.6mgP23 national0.0%
- ThiosulfateS₂O₃²⁻1.8mgP82 national0.0%
- SS²⁻1.2mgP97 national
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃111mgP59 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂52.9mgP89 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂249mgP90 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national
- ArsenicAs< —mg
- CadmiumCd< —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.