A sodium-led bicarbonate spring, the classic jūsō-sen style, which the certificate still records under its older name of a pure sodium-bicarbonate spring. Weakly alkaline and soft on the skin, it sits squarely in Japan's beauty-water family. Dissolved solids of 1,317 mg/kg give the water moderate body without much bite, and a generous 255 mg/kg of metasilicic acid leaves the kind of thin 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⁺207mgP52 national68%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺34.2mgP49 national13%
- Magnesiummild; a laxative if you drink itMg²⁺18.6mgP70 national12%
- Potassiumminor; matters for drinking limitsK⁺30.3mgP75 national6%
- LithiumLi⁺1.3mgP85 national1%
- AmmoniumNH₄⁺0.30mgP40 national0.1%
- ManganeseMn²⁺0.30mgP53 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- Iron (II)Fe²⁺0.20mgP39 national0.1%
- BariumBa²⁺0.20mgP74 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻650mgP82 national79%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻56.7mgP29 national12%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻56.9mgP42 national9%
- ThiosulfateS₂O₃²⁻0.40mgP62 national0.1%
- Hydrogen phosphateHPO₄²⁻0.30mgP60 national0.1%
- BromideBr⁻0.20mgP30 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃255mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.4mgP32 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- ArsenicAs0.14mgP70 national
- 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.