A blended simple radon spring drawing from the Torikai Rotenburo and Torikai No.2 wells, served at 42.5 °C. Weakly alkaline and lightly mineralized (482 mg/kg total dissolved solids), so the water reads soft on the skin. Radon registers at 274 Bq/kg — squarely in the 'hormesis' tradition that Japan's radon spas are known for.
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⁺118mgP31 national88%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.0mgP30 national10%
- Magnesiummild; a laxative if you drink itMg²⁺2.4mgP37 national3%
- Potassiumminor; matters for drinking limitsK⁺3.4mgP28 national2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻82.1mgP36 national40%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻76.1mgP52 national27%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻91.5mgP40 national26%
- Fluoridetrace; drinking limits applyF⁻6.9mgP87 national6%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃81.0mgP49 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP19 national
- Metaarsenious acidHAsO₂0.10mgP59 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.9mgP34 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- Radonfaint radon — studied for joint pain (hormesis)Rn274mgP82 national
- ArsenicAs0.09mgP71 national
- 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.