A naturally hot source rising in the Tsurumi district of Beppu, Oita, and hot enough to sit among Japan's hottest natural springs. The water is a simple thermal spring, hypotonic and neutral, with light mineralization of just 0.53 g/kg, so it feels gentle and unassertive on the skin. What lifts it out of the ordinary is silica: metasilicic acid at 162 mg/kg leaves the kind of thin moisturizing film that puts this water squarely in Japan's beauty-water family.
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⁺70.7mgP21 national66%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺18.1mgP36 national19%
- Magnesiummild; a laxative if you drink itMg²⁺6.0mgP55 national11%
- Potassiumminor; matters for drinking limitsK⁺6.6mgP38 national4%
- AluminumAl³⁺0.20mgP64 national0.5%
- FeTotalFe0.20mgP44 national0.1%
- BariumBa²⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- StrontiumSr²⁺0.0000mg0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻164mgP53 national55%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻82.9mgP50 national35%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻16.1mgP16 national9%
- CarbonateCO₃²⁻0.40mgP14 national0.3%
- FluorideF⁻0.20mgP16 national0.2%
- NitrateNO₃⁻0.60mgP66 national0.2%
- ThiosulfateS₂O₃²⁻0.50mgP62 national0.2%
- Hydrogen phosphateHPO₄²⁻0.20mgP53 national0.1%
- AsO2AsO₂⁻0.0000mg0.0%
- BO2BO₂⁻0.0000mg0.0%
- BromideBr⁻0.0000mg0.0%
- H2PO4H₂PO₄⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- HSiO3HSiO₃⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- SS²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃162mgP75 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.70mgP10 national
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.7mgP36 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —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.