硫黄泉Iou Spring
A simple sulfur spring (hydrogen sulfide subtype) — pale, clear water with the unmistakable rotten-egg note of H₂S. Weakly acidic and gentle on the skin, with metasilicic acid at 127 mg/kg leaving the kind of silica film that makes skin feel noticeably smoother afterwards. The cert was sampled at the bath inlet of a free-flowing source delivering 148 L/min.
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⁺78.7mgP25 national62%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺27.1mgP44 national24%
- Magnesiummild; a laxative if you drink itMg²⁺4.3mgP45 national7%
- Potassiumminor; matters for drinking limitsK⁺13.2mgP51 national6%
- AmmoniumNH₄⁺0.36mgP41 national0.4%
- ManganeseMn²⁺0.20mgP45 national0.2%
- StrontiumSr²⁺0.20mgP44 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻154mgP51 national48%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻120mgP58 national47%
- Hydrogen sulfidesulfur character — antibacterialHS⁻10.5mgP95 national6%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻7.4mgP10 national4%
- Fluoridetrace; drinking limits applyF⁻1.3mgP52 national1%
- ThiosulfateS₂O₃²⁻0.50mgP64 national0.2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃127mgP64 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.8mgP29 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂295mgP92 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S3.9mgP84 national
- ArsenicAs0.0040mgP11 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