Yunotsubo Onsen rises in the Kawakami district of Yufuin as a simple thermal spring: weakly alkaline, soft on the skin, and lightly mineralized at 685 mg/kg of dissolved solids, so it stays gentle enough for a long, unhurried soak. What lifts it above plain water is silica — metasilicic acid at 214 mg/kg, generous enough to leave the thin moisturizing film that places this source squarely in Japan's beauty-water family. The certificate's sample was drawn at the storage-tank outlet rather than at the wellhead, so the figures describe the water close to the state in which it reaches 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⁺118mgP34 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.6mgP25 national8%
- Potassiumminor; matters for drinking limitsK⁺17.0mgP56 national7%
- Magnesiummild; a laxative if you drink itMg²⁺3.2mgP40 national4%
- LithiumLi⁺0.20mgP38 national0.5%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻184mgP56 national47%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻77.0mgP33 national34%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻57.8mgP42 national19%
- FluorideF⁻0.40mgP29 national0.3%
- BromideBr⁻0.20mgP29 national0.0%
- Hydrogen phosphateHPO₄²⁻0.20mgP53 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃214mgP89 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.2mgP24 national
- Metaarsenious acidHAsO₂0.40mgP78 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂9.7mgP40 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.24mgP83 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.