A blend of two wells at Beppu Kamenoi Hotel, classified as a simple thermal spring. Hot at 43.5 °C, weakly alkaline at pH 7.6, and soft on the skin. Total dissolved solids are light at 721.8 mg/kg, but a generous 195.6 mg/kg of metasilicic acid leaves the kind of silica film that puts it 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
Take care
- 温泉分析書別表5に記載
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⁺77.1mgP24 national47%
- Magnesiummild; a laxative if you drink itMg²⁺21.9mgP71 national25%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺33.8mgP48 national24%
- Potassiumminor; matters for drinking limitsK⁺9.2mgP44 national3%
- LithiumLi⁺0.50mgP55 national1.0%
- Iron (II)Fe²⁺0.80mgP59 national0.4%
- ManganeseMn²⁺0.30mgP53 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻242mgP60 national54%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻62.4mgP31 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻74.9mgP50 national21%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃196mgP84 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.7mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.8mgP34 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.05mgP48 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- 温泉分析書別表5に記載
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.