A sodium- and magnesium-led bicarbonate spring, with the soda-water softness of the jūsō-sen family layered with magnesium body. Weakly alkaline at pH 7.6 and soft on the skin, it sits in the Japanese beauty-water family. A generous 243 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards. At the wellhead the water is pale yellow and clear, with a faint metallic, iron-like note on the tongue and nose.
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⁺126mgP35 national53%
- Magnesiummild; a laxative if you drink itMg²⁺27.5mgP74 national22%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺41.2mgP54 national20%
- Potassiumminor; matters for drinking limitsK⁺11.6mgP48 national3%
- LithiumLi⁺0.80mgP70 national1%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺3.1mgP77 national1%
- ManganeseMn²⁺0.30mgP53 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻415mgP72 national67%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻59.0mgP30 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻77.2mgP50 national16%
- Hydrogen phosphateHPO₄²⁻0.30mgP60 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃243mgP94 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.6mgP27 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂13.2mgP46 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- 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.