A sodium-led bicarbonate spring of the classic jūsō-sen style, rising at a hot 55.2 °C in Kirishima. Weakly alkaline (pH 7.9) and soft on the skin, it sits squarely in Japan's beauty-water family. A generous 207.9 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother after a soak.
Good for
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
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
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⁺360mgP66 national93%
- Potassiumminor; matters for drinking limitsK⁺34.3mgP78 national5%
- MagnesiumMg²⁺1.8mgP33 national0.9%
- CalciumCa²⁺2.4mgP9 national0.7%
- LithiumLi⁺0.20mgP40 national0.2%
- AluminumAl³⁺0.10mgP54 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻867mgP87 national83%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻97.1mgP39 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻10.0mgP21 national1%
- FluorideF⁻0.60mgP38 national0.2%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃208mgP87 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂10.6mgP58 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP58 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0040mgP11 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.