A sodium-led bicarbonate spring; the certificate records its legacy name as a pure jūsō-sen, and it reads that way: mildly alkaline, soft on the skin, with the smooth soda-water touch the style is known for. Dissolved solids sit at a moderate 1,364 mg/kg, enough mineral body to feel without weight. The standout figure is metasilicic acid at 260 mg/kg, a generous level that leaves the thin silica film Japan's beauty-water family is named for.
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⁺211mgP53 national66%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.2mgP51 national14%
- Magnesiummild; a laxative if you drink itMg²⁺21.3mgP71 national13%
- Potassiumminor; matters for drinking limitsK⁺34.7mgP78 national6%
- LithiumLi⁺1.3mgP85 national1%
- AmmoniumNH₄⁺0.30mgP40 national0.1%
- ManganeseMn²⁺0.40mgP59 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- BariumBa²⁺0.0000mg0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻646mgP82 national75%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻81.6mgP36 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻60.3mgP44 national9%
- FluorideF⁻0.40mgP31 national0.1%
- Hydrogen phosphateHPO₄²⁻0.30mgP60 national0.1%
- ThiosulfateS₂O₃²⁻0.30mgP58 national0.1%
- BromideBr⁻0.20mgP30 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃260mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.5mgP44 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP58 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.15mgP72 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.