An acidic iron-bearing sulfate spring rising at Unzen's Hachiman Jigoku. At pH 3.4 the water is faintly sour and milky-white with a pale yellow cast, carrying the characteristic rotten-egg note of H₂S. Sulfate dominates the dissolved load, and a generous 184 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel 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.
- Aluminumshows up in acidic volcanic waterAl³⁺42.5mgP93 national34%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺4.0mgP76 national28%
- AmmoniumNH₄⁺26.1mgP99 national10%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺10.5mgP3 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺46.2mgP56 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻644mgP91 national93%
- HSO4HSO484.4mgP66 national6%
- ChlorideCl⁻6.9mgP10 national0.5%
- ThiosulfateS₂O₃²⁻2.2mgP87 national0.3%
- FluorideF⁻5.3mgP79 national0.1%
- BromideBr⁻0.80mgP54 national0.1%
- IodideI⁻0.40mgP66 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃184mgP82 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.6mgP16 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S27.9mgP98 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂6.9mgP33 national
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