A sodium-led bicarbonate-chloride spring — a jūsō-sen base with the salty edge of a chloride sub-type layered on top. Neutral pH, a faint hydrogen sulfide note on the nose, and 64.7 °C at the wellhead. Metasilicic acid at a generous 119.5 mg/kg leaves a thin silica film that smooths the skin, while 5,704 mg/kg of dissolved solids gives the water noticeable body.
Good for
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
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
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
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⁺1,586mgP90 national90%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺60.5mgP60 national4%
- Potassiumminor; matters for drinking limitsK⁺87.4mgP91 national3%
- Magnesiummild; a laxative if you drink itMg²⁺24.4mgP73 national3%
- ManganeseMn²⁺0.40mgP59 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,434mgP98 national51%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,314mgP83 national48%
- SulfateSO₄²⁻19.3mgP30 national0.5%
- FluorideF⁻4.9mgP78 national0.3%
- ThiosulfateS₂O₃²⁻1.0mgP72 national0.0%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃120mgP61 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂52.8mgP89 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂82.5mgP76 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.