A hot (48.8 °C) bicarbonate-sulfate spring led by sodium, magnesium and calcium. The wellhead water is colorless and clear with a faintly metallic taste, but on standing it develops a strong yellow turbidity — a glimpse of the iron it carries. Metasilicic acid at a generous 211 mg/kg 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
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
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⁺290mgP61 national37%
- Magnesiummild; a laxative if you drink itMg²⁺120mgP92 national29%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺192mgP84 national28%
- Potassiumminor; matters for drinking limitsK⁺69.1mgP89 national5%
- LithiumLi⁺0.60mgP61 national0.2%
- AmmoniumNH₄⁺0.80mgP61 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,249mgP92 national57%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻420mgP83 national24%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻242mgP58 national19%
- H2PO4H₂PO₄⁻0.10mgP31 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.40mgP15 national
- Fluoridetrace; drinking limits applyF⁻0.02mgP3 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃211mgP88 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.9mgP62 national
- Metaarsenious acidHAsO₂0.40mgP81 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂750mgP98 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.30mgP87 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.
- 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.