A gentle simple thermal spring rising in the Kawakami district of Yufuin, Yufu City. Weakly alkaline and lightly mineralized at just 0.34 g/kg of dissolved solids, the water is colorless, clear, tasteless and odorless — about as unintrusive as onsen water gets. What lifts it is the silica: metasilicic acid at 188.0 mg/kg is generous enough to place this water in Japan's beauty-water family, leaving the thin mineral 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⁺38.2mgP8 national91%
- Potassiumminor; matters for drinking limitsK⁺6.6mgP38 national9%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻72.7mgP37 national61%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻18.6mgP16 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻8.9mgP20 national10%
- Fluoridetrace; drinking limits applyF⁻0.60mgP36 national2%
- NitrateNO₃⁻0.80mgP72 national0.5%
- Hydrogen phosphateHPO₄²⁻0.60mgP74 national0.5%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃188mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.0mgP12 national
- Metaarsenious acidHAsO₂0.10mgP53 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.4mgP28 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.08mgP61 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.