松葉温泉Matsuba Onsen
A radon-bearing weak radioactive spring. Weakly alkaline at pH 8.4 and only lightly mineralized at 729 mg/kg, so the water itself feels gentle on the skin. The radon load of 236.8 Bq/kg (17.6 Mache) is what makes the source distinctive — well into the 'hormesis' tradition of radon bathing. Heated, recirculated and chlorine-disinfected before reaching the 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⁺219mgP56 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.0mgP24 national5%
- PotassiumK⁺0.40mgP4 national0.1%
- StrontiumSr²⁺0.20mgP41 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- LithiumLi⁺< —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⁻240mgP60 national64%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻158mgP53 national24%
- Fluoridetrace; drinking limits applyF⁻14.0mgP95 national7%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻21.6mgP68 national5%
- SulfateSO₄²⁻2.2mgP10 national0.5%
- Bromidetrace, sea-originBr⁻0.20mgP28 national
- Iodideyellows in air; helps blood lipidsI⁻0.10mgP35 national
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂36.9mgP84 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃26.9mgP10 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn237mgP73 national
- 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.