A sodium-magnesium bicarbonate spring with a chloride edge, filed under the older soda-water (jūsō-sen) classification. Neutral in reaction and moderately mineralized at about 1,134 mg/kg, it is soft on the skin and sits squarely in Japan's beauty-water family. A generous 193 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards. At the wellhead the water is colorless and clear with only the faintest metallic taste; the certificate records that five hours later it clouds very slightly and picks up an iron note, the signature of the small amount of dissolved iron it carries.
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
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺146mgP39 national52%
- Magnesiummild; a laxative if you drink itMg²⁺36.3mgP80 national24%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺44.6mgP56 national18%
- Potassiumminor; matters for drinking limitsK⁺21.2mgP65 national4%
- LithiumLi⁺1.0mgP74 national1%
- Iron (II)Fe²⁺1.3mgP65 national0.4%
- StrontiumSr²⁺0.30mgP55 national0.1%
- BariumBa²⁺0.20mgP74 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻537mgP77 national70%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻95.0mgP39 national21%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻53.5mgP41 national9%
- BromideBr⁻0.30mgP36 national0.0%
- ThiosulfateS₂O₃²⁻0.10mgP45 national0.0%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃193mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.8mgP36 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂43.6mgP67 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.11mgP65 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.