A simple weakly-radioactive spring carrying enough radon to fall in the "hormesis" tradition, with radon measured at 330 Bq/kg. Weakly alkaline at pH 8.25, it is soft on the skin, colorless, transparent and odorless. Mineralization is light at 429 mg/kg, making for a gentle, low-stimulus bath.
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⁺112mgP33 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺31.1mgP46 national24%
- PotassiumK⁺0.80mgP11 national0.3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (III)Fe³⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- AmmoniumNH₄⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻166mgP50 national73%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻78.3mgP38 national20%
- Fluoridetrace; drinking limits applyF⁻7.9mgP87 national7%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.20mgP3 national
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃31.3mgP14 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.5mgP15 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.