A simple sulfur spring of the hydrogen sulfide subtype, welling up at 87 °C from one of Japan's hottest natural sources high in the Hachimantai mountains. At the source the water is milky-white and faintly turbid, carrying the unmistakable rotten-egg note of H₂S. It runs weakly acidic at pH 3.7, with lightly mineralized chemistry (177 mg/kg dissolved solids) but a clear sulfur signature — free H₂S at 4.12 mg/kg and free CO₂ at 52.9 mg/kg. Classified under the older name 'tanjun ryūkasuiso-sen' (simple hydrogen sulfide spring), it's the kind of high-mountain sulfur water that needs no dilution or reheating to bathe in.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.5mgP23 national32%
- Magnesiummild; a laxative if you drink itMg²⁺2.9mgP38 national18%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.20mgP59 national15%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺2.9mgP75 national8%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺1.8mgP0 national6%
- Aluminumshows up in acidic volcanic waterAl³⁺0.80mgP77 national3%
- AmmoniumNH₄⁺0.60mgP53 national2%
- PotassiumK⁺0.30mgP3 national0.6%
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻58.1mgP43 national96%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.7mgP2 national4%
- ThiosulfateS₂O₃²⁻0.10mgP45 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Fluoridetrace; drinking limits applyF⁻< —mg
- H2PO4H₂PO₄⁻< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃40.0mgP21 national
- Metaarsenious acidHAsO₂< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂52.9mgP70 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S4.1mgP84 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0032mgP91 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —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