A sodium-led bicarbonate spring — the classic jūsō-sen style, soft on the skin and once classified under the old name 'pure soda spring'. Weakly alkaline, it sits comfortably in Japan's beauty-water family. A generous 208 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother. The water is colorless, clear and all but odorless, gentle to soak in.
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⁺245mgP57 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺36.5mgP50 national12%
- Magnesiummild; a laxative if you drink itMg²⁺22.0mgP72 national12%
- Potassiumminor; matters for drinking limitsK⁺18.8mgP61 national3%
- LithiumLi⁺1.5mgP88 national1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- BariumBa²⁺0.20mgP74 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻583mgP79 national65%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻81.3mgP35 national16%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻48.0mgP91 national11%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻54.7mgP41 national8%
- Hydrogen phosphateHPO₄²⁻2.9mgP91 national0.4%
- FluorideF⁻0.20mgP17 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃208mgP87 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.7mgP35 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- ArsenicAs0.14mgP70 national
- CadmiumCd< —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.