An acidic, sulfur-bearing sodium-magnesium sulfate spring. At pH 2.4 the water is sharp enough to prickle on contact, and free hydrogen sulfide runs to 108.1 mg/kg — the unmistakable rotten-egg note hangs over the baths. The source emerges at 72.3 °C, among Japan's hottest, so it is diluted with water to bring the tubs down to bathing temperature; nothing is recirculated and no disinfectant is added, the posting noting that the water's own strong acidity makes disinfection unnecessary. A generous 124.9 mg/kg of metasilicic acid leaves a thin film on the skin. The hotel's own guest information points to this water for fatigue recovery and cold sensitivity.
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⁺90.4mgP25 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺41.0mgP53 national
- Magnesiummild; a laxative if you drink itMg²⁺35.6mgP80 national
- Potassiumminor; matters for drinking limitsK⁺23.9mgP69 national
- Aluminumshows up in acidic volcanic waterAl³⁺6.3mgP86 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺4.0mgP75 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺3.8mgP79 national
- Manganesetrace; companion to iron springsMn²⁺2.6mgP93 national
- AmmoniumNH₄⁺0.60mgP52 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻549mgP89 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻77.0mgP32 national
- HSO4HSO473.6mgP65 national
- Fluoridetrace; drinking limits applyF⁻1.1mgP47 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃125mgP64 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.3mgP42 national
- H2SO4FreeH2SO4Free0.70mgP70 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S108mgP100 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂30.8mgP60 national
- ArsenicAs0.0060mgP18 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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