A sodium-led chloride spring with bicarbonate softness layered in, emerging hot at 44.6 °C. Weakly alkaline and soft on the skin, it is colorless and clear but carries a faint salty, slightly metallic taste and a whiff of sulfur. With dissolved solids around 1,896 mg/kg it is moderately mineralized, giving the water a noticeable warming body.
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⁺601mgP79 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺10.8mgP27 national2%
- PotassiumK⁺9.6mgP45 national0.9%
- LithiumLi⁺1.0mgP74 national0.5%
- AmmoniumNH₄⁺2.1mgP81 national0.4%
- MagnesiumMg²⁺1.5mgP29 national0.4%
- StrontiumSr²⁺1.9mgP85 national0.1%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- HydrogenH⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.20mgP64 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻642mgP77 national66%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻568mgP79 national34%
- FluorideF⁻1.2mgP50 national0.2%
- BromideBr⁻1.1mgP62 national0.0%
- CarbonateCO₃²⁻0.0000mg0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- SulfateSO₄²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃37.6mgP20 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.0mgP71 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.6mgP23 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.02mgP100 national
- CadmiumCd0.0000mg
- ArsenicAs< —mg
- CopperCu< —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.