A sodium-led bicarbonate spring with sulfate and chloride layered in, drawn very hot at 72.8 °C — among Japan's hottest natural sources. The pH is neutral so the touch is unforced, but an unusually high 348 mg/kg of metasilicic acid leaves the kind of silica film associated with Japan's beauty-water family. At the source the water carries a very faint yellow-brown tint and a touch of turbidity, giving it visible body.
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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⁺177mgP46 national64%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺29.2mgP45 national12%
- Magnesiummild; a laxative if you drink itMg²⁺17.2mgP69 national12%
- Potassiumminor; matters for drinking limitsK⁺49.7mgP85 national11%
- LithiumLi⁺0.80mgP70 national1.0%
- Iron (II)Fe²⁺1.0mgP62 national0.2%
- AmmoniumNH₄⁺0.40mgP43 national0.2%
- ManganeseMn²⁺0.30mgP53 national0.2%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻270mgP62 national39%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻197mgP65 national36%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻101mgP40 national25%
- FluorideF⁻0.30mgP24 national0.1%
- BromideBr⁻0.40mgP39 national0.0%
- H2PO4H₂PO₄⁻0.40mgP83 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃348mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.0mgP61 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂131mgP83 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.