A simple weak-radioactive spring of the radon-bearing "hormesis" tradition, carrying a notable 159.1 Bq/kg of radon. Mineralization is light at just 0.17 g/kg, and the water is mildly alkaline and soft on the skin. Almost colorless, transparent, tasteless and odorless, it is a gentle, low-mineral water whose defining feature is its radon content.
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⁺41.7mgP10 national91%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.2mgP12 national8%
- Potassiumminor; matters for drinking limitsK⁺0.90mgP12 national1%
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP14 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.06mgP31 national
- LithiumLi⁺0.06mgP18 national
- StrontiumSr²⁺0.04mgP14 national
- BariumBa²⁺0.0040mgP28 national
- Manganesetrace; companion to iron springsMn²⁺0.0020mgP14 national
- CopperCu²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻71.4mgP35 national60%
- Fluoridetrace; drinking limits applyF⁻8.4mgP88 national22%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻7.6mgP11 national11%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻4.5mgP15 national5%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻1.5mgP22 national3%
- NitrateNO₃⁻0.30mgP43 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃27.4mgP10 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.1mgP20 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.50mgP14 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.