A radon-bearing 'simple weakly radioactive' spring — a small but real dose of Rn places it in the hormesis tradition. Alkaline at pH 9.34, the water has the slippery beauty-water touch despite a light overall mineralization of just 0.242 g/kg. Radon registers at roughly 220 Bq/kg (16.4 Mache units), well above the threshold for a radioactive spring; the source emerges hot at 43 °C.
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⁺50.9mgP14 national94%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.7mgP11 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻33.5mgP24 national29%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻15.4mgP10 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻74.9mgP96 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃42.2mgP25 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂NDmg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.04mgP53 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.