A simple sulfur spring of the hydrogen-sulfide type, drawn on the Nakao Kogen plateau in the Okuhida hot-spring district. At 87.4 °C at the wellhead it ranks among Japan's hottest natural sources, and while the water itself is colorless, transparent and tasteless, it carries the characteristic faint rotten-egg note of H₂S — 2.0 mg/kg of free hydrogen sulfide alongside a trace of bisulfide ion. Mineralization is light at 288 mg/kg of dissolved solids and the pH is neutral, so the feel on the skin is gentle rather than sharp. The certificate's sample was taken from the storage tank rather than at the wellhead.
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
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⁺46.3mgP12 national61%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺13.4mgP30 national20%
- Potassiumminor; matters for drinking limitsK⁺16.1mgP55 national13%
- Magnesiummild; a laxative if you drink itMg²⁺1.2mgP28 national3%
- LithiumLi⁺0.60mgP59 national3%
- AmmoniumNH₄⁺0.30mgP39 national0.5%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻118mgP47 national61%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻38.6mgP24 national34%
- Fluoridetrace; drinking limits applyF⁻2.6mgP63 national4%
- Hydrogen sulfideHS⁻0.40mgP50 national0.3%
- NitrateNO₃⁻0.20mgP42 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃44.0mgP24 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.4mgP37 national
- Metaarsenious acidHAsO₂0.20mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂59.2mgP72 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S2.0mgP80 national
- ArsenicAs0.13mgP70 national
- LeadPb0.01mgP50 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.
- 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