A simple thermal source ('Keihanna Gakken Tennen Onsen') with light overall mineralization — 0.83 g/kg dissolved solids — yet weakly alkaline and soft on the skin, sitting squarely in the Japanese 'beauty-water' family. Generous metasilicic acid (86.6 mg/kg) leaves a thin silica film that smooths the skin, and an unusually high radon content of 90.9 Bq/kg places it in the 'hormesis' radon tradition — a rare pairing in a single-named simple spring.
Good for
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
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⁺187mgP48 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.0mgP21 national4%
- Magnesiummild; a laxative if you drink itMg²⁺2.3mgP36 national2%
- Potassiumminor; matters for drinking limitsK⁺4.2mgP32 national1%
- AluminumAl³⁺0.40mgP74 national0.5%
- Iron (II)Fe²⁺0.50mgP52 national0.2%
- AmmoniumNH₄⁺0.10mgP24 national0.1%
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻509mgP76 national95%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8.6mgP12 national3%
- Fluoridetrace; drinking limits applyF⁻2.6mgP64 national2%
- CarbonateCO₃²⁻2.4mgP26 national0.9%
- Hydrogen sulfideHS⁻0.30mgP47 national0.1%
- SulfateSO₄²⁻0.30mgP4 national0.1%
- Bromidetrace, sea-originBr⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃86.6mgP49 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.2mgP66 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.8mgP29 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.
- 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.