An acidic, sulfur-bearing sulfate spring blending the Radium Hokko, Tetsuyu No.1, Nigayu No.3 and Ubayu wells. A hot source at 69.2 degrees C, it carries the unmistakable rotten-egg note of H₂S, with abundant free hydrogen sulfide. Metasilicic acid runs to 160 mg/kg, leaving the kind of silica film that makes skin feel noticeably smoother. Because the source runs hot it is diluted with spring water (the Ubayu well alone is left undiluted), with no heating, recirculation, disinfection or bath additives.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
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
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
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
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⁺107mgP31 national
- Magnesiummild; a laxative if you drink itMg²⁺61.1mgP87 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺36.7mgP50 national
- Potassiumminor; matters for drinking limitsK⁺20.8mgP64 national
- Aluminumshows up in acidic volcanic waterAl³⁺8.3mgP90 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺4.1mgP78 national
- Manganesetrace; companion to iron springsMn²⁺4.0mgP97 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺3.4mgP75 national
- AmmoniumNH₄⁺0.30mgP40 national
- ZincZn²⁺0.01mgP41 national
- Iron (III)Fe³⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻616mgP90 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻103mgP40 national
- HSO4HSO470.3mgP64 national
- Fluoridetrace; drinking limits applyF⁻1.0mgP47 national
- Iodideyellows in air; helps blood lipidsI⁻0.70mgP75 national
- Bromidetrace, sea-originBr⁻0.50mgP47 national
- H2PO4H₂PO₄⁻0.40mgP83 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃160mgP73 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.2mgP44 national
- H2SO4FreeH2SO4Free0.60mgP71 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂131mgP83 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S45.8mgP99 national
- ArsenicAs0.0050mgP15 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.
- 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.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official