A hot (78.9 °C) simple thermal spring in southern Aso, among Japan's hotter natural sources. Weakly acidic at pH 3.5 with a faint rotten-egg note of H₂S — gentle on the body despite the heat. Generous metasilicic acid at 183.5 mg/kg 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⁺82.7mgP24 national58%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺18.4mgP37 national15%
- Potassiumminor; matters for drinking limitsK⁺32.8mgP77 national14%
- Magnesiummild; a laxative if you drink itMg²⁺4.5mgP48 national6%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.30mgP59 national5%
- LithiumLi⁺0.80mgP68 national2%
- AmmoniumNH₄⁺0.40mgP43 national0.4%
- AluminumAl³⁺0.20mgP65 national0.4%
- ManganeseMn²⁺0.50mgP66 national0.3%
- Iron (II)Fe²⁺0.20mgP38 national0.1%
- ZincZn²⁺0.02mgP44 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻204mgP65 national69%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻65.2mgP30 national30%
- FluorideF⁻0.70mgP38 national0.6%
- HSO4HSO42.2mgP51 national0.4%
- BromideBr⁻0.20mgP28 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃184mgP82 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.5mgP32 national
- Metaarsenious acidHAsO₂0.20mgP67 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂23.8mgP54 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.70mgP72 national
- 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.