A sodium-led bicarbonate spring with a salty chloride edge, of the classic jūsō-sen (soda-water) family. At the source the water carries a faint yellow-brown tint, a slightly salty taste, and a faint hydrogen sulfide note. With 153.7 mg/kg of metasilicic acid it leaves the kind of 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
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⁺257mgP58 national66%
- Magnesiummild; a laxative if you drink itMg²⁺31.9mgP76 national16%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺42.9mgP55 national13%
- Potassiumminor; matters for drinking limitsK⁺28.7mgP73 national4%
- AmmoniumNH₄⁺1.9mgP80 national0.7%
- Iron (II)Fe²⁺0.50mgP52 national0.1%
- StrontiumSr²⁺0.40mgP62 national0.1%
- BariumBa²⁺0.20mgP74 national
- Manganesetrace; companion to iron springsMn²⁺0.20mgP45 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻673mgP83 national64%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻215mgP56 national35%
- Bromidetrace, sea-originBr⁻0.60mgP49 national
- Iodideyellows in air; helps blood lipidsI⁻0.60mgP73 national
- Fluoridetrace; drinking limits applyF⁻0.20mgP17 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.20mgP40 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃154mgP69 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP19 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂28.7mgP59 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.