A hot (61 °C) sodium bicarbonate spring — the classic jūsō-sen style, soft on the skin. Alkaline at pH 8.5, it sits comfortably in Japan's beauty-water family. A generous 256 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards. The brown clear water is the signature look of a plant-derived moor spring.
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⁺269mgP58 national93%
- Potassiumminor; matters for drinking limitsK⁺14.5mgP52 national3%
- AmmoniumNH₄⁺6.1mgP93 national3%
- CalciumCa²⁺2.2mgP8 national0.9%
- LithiumLi⁺0.40mgP50 national0.5%
- MagnesiumMg²⁺0.40mgP18 national0.2%
- AluminumAl³⁺0.20mgP64 national0.2%
- Iron (II)Fe²⁺0.10mgP30 national0.0%
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻610mgP80 national83%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻45.0mgP90 national12%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻12.3mgP15 national3%
- Fluoridetrace; drinking limits applyF⁻2.9mgP65 national1%
- Hydrogen phosphateHPO₄²⁻2.7mgP91 national0.5%
- SulfateSO₄²⁻2.6mgP11 national0.4%
- NitrateNO₃⁻0.30mgP43 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃256mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.4mgP26 national
- Metaarsenious acidHAsO₂0.50mgP83 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn0.10mgP9 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.