A blend: water from two wellheads in Yufuin is brought together in a 40-tonne tank, and the certificate takes its sample there, after mixing. What comes out is an alkaline simple thermal spring — lightly mineralized and gentle, with nothing sharp about it — yet hot enough at the source to sit among Japan's hottest natural waters. The alkalinity gives it the slippery feel of Japan's beauty-water family, and a generous 181 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards.
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⁺123mgP35 national82%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.8mgP27 national9%
- Potassiumminor; matters for drinking limitsK⁺11.5mgP47 national4%
- Magnesiummild; a laxative if you drink itMg²⁺3.2mgP41 national4%
- LithiumLi⁺0.30mgP41 national0.6%
- StrontiumSr²⁺0.10mgP24 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻204mgP58 national49%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻82.4mgP36 national34%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻32.9mgP36 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻13.2mgP55 national6%
- FluorideF⁻0.40mgP28 national0.3%
- BromideBr⁻0.20mgP28 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃181mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.5mgP32 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.06mgP57 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.