A sodium-led chloride and bicarbonate spring emerging at a hot 44 °C. Weakly alkaline (pH 8.1) and soft on the skin, with a faintly metallic taste and a light hydrogen sulfide note layered over the chloride body. Moderately mineralized at 1,276 mg/kg dissolved solids — the chloride backbone keeps warmth in, while the bicarbonate sub-type adds a smoother finish.
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⁺378mgP65 national89%
- Magnesiummild; a laxative if you drink itMg²⁺10.6mgP62 national5%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺10.9mgP27 national3%
- Potassiumminor; matters for drinking limitsK⁺18.1mgP60 national3%
- LithiumLi⁺0.20mgP37 national0.2%
- ZincZn²⁺0.20mgP85 national0.1%
- Iron (II)Fe²⁺0.10mgP32 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- BariumBa²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- StrontiumSr²⁺0.20mgP39 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻381mgP69 national61%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻424mgP70 national39%
- Hydrogen sulfideHS⁻0.60mgP57 national0.2%
- FluorideF⁻0.30mgP25 national0.1%
- BromideBr⁻1.1mgP59 national0.1%
- Hydrogen phosphateHPO₄²⁻0.30mgP60 national0.1%
- AsO2AsO₂⁻0.0000mg0.0%
- BO2BO₂⁻0.0000mg0.0%
- CarbonateCO₃²⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- SS²⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- SulfateSO₄²⁻0.0000mg0.0%
- Iodideyellows in air; helps blood lipidsI⁻0.60mgP73 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃41.9mgP26 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.3mgP47 national
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.8mgP49 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP52 national
- CopperCu0.02mgP83 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.