A simple thermal spring rising in Yufuin. Sodium and bicarbonate lead the ion table, but with only about 0.7 g/kg of dissolved solids the water stays lightly mineralized and gentle on the body. It is mildly alkaline and soft on the skin, and a generous 198 mg/kg of metasilicic acid puts it in Japan's beauty-water family — the silica leaves a thin film that makes skin feel noticeably smoother once you are out.
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⁺132mgP36 national81%
- Potassiumminor; matters for drinking limitsK⁺17.5mgP58 national6%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.0mgP24 national6%
- Magnesiummild; a laxative if you drink itMg²⁺5.2mgP51 national6%
- LithiumLi⁺0.20mgP38 national0.4%
- ManganeseMn²⁺0.20mgP45 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻176mgP55 national40%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻78.1mgP33 national31%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻59.6mgP43 national17%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻24.6mgP77 national11%
- FluorideF⁻0.30mgP22 national0.3%
- BromideBr⁻0.20mgP29 national0.0%
- Hydrogen phosphateHPO₄²⁻0.10mgP33 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃198mgP85 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.3mgP31 national
- Metaarsenious acidHAsO₂0.20mgP67 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.10mgP65 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.