A blend of two wells, Ogama and Ontake, arriving at a scalding 76.0 °C — among Japan's hottest natural sources. The certificate classifies it as an alkaline simple sulfur spring: the water runs colorless and clear, yet the rotten-egg note of hydrogen sulfide is unmistakable at the spout. At pH 8.5 at the wellhead (8.83 in the lab) and just 0.998 g/kg of dissolved solids, it is light on minerals but sits squarely in Japan's beauty-water family, with a soft, slippery feel on the skin. Metasilicic acid at 119 mg/kg leaves the kind of silica film that makes skin noticeably smoother afterwards.
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⁺189mgP49 national64%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺86.8mgP69 national34%
- Potassiumminor; matters for drinking limitsK⁺6.6mgP38 national1%
- AmmoniumNH₄⁺1.0mgP65 national0.4%
- StrontiumSr²⁺0.50mgP68 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻456mgP85 national71%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻89.1mgP38 national19%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻20.4mgP67 national5%
- Hydrogen sulfidesulfur character — antibacterialHS⁻15.7mgP97 national4%
- HydroxideOH⁻3.9mgP97 national2%
- FluorideF⁻1.2mgP49 national0.5%
- BromideBr⁻0.30mgP37 national0.0%
- NitrateNO₃⁻0.20mgP42 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃119mgP61 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.8mgP48 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.50mgP71 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- 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