A sodium-magnesium bicarbonate spring in the soda-water tradition — the cert pairs the modern bicarbonate classification with the older 'earth-bearing soda spring' (gansa-rui jūsō-sen) name. Neutral pH and soft on the skin, it sits within Japan's classic beauty-water family. A generous 178 mg/kg of metasilicic acid leaves the kind of fine silica film that makes skin feel noticeably smoother afterwards.
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⁺182mgP47 national57%
- Magnesiummild; a laxative if you drink itMg²⁺34.7mgP80 national20%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.7mgP53 national15%
- Potassiumminor; matters for drinking limitsK⁺38.2mgP80 national7%
- LithiumLi⁺1.1mgP76 national1%
- Iron (II)Fe²⁺0.40mgP48 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- AmmoniumNH₄⁺0.20mgP30 national0.1%
- BariumBa²⁺0.30mgP77 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻624mgP81 national74%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻82.4mgP36 national17%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻63.7mgP45 national10%
- FluorideF⁻0.20mgP17 national0.1%
- BromideBr⁻0.20mgP30 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃178mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.0mgP37 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂36.1mgP63 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.17mgP78 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.