An acidic sulfate spring that is also sulfur- and iron-bearing, emerging hot at 52 °C. At pH 2.3 it is sharply acidic with a prickly feel on the skin, carrying the unmistakable rotten-egg note of H₂S along with a sour, faintly metallic taste. The iron content gives it a faintly yellow, slightly turbid cast, and at 180 mg/kg of metasilicic acid it leaves skin noticeably smoother. A distinctly characterful Unzen water.
Good for
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
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
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.
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺5.1mgP78 national38%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺58.1mgP60 national22%
- Aluminumshows up in acidic volcanic waterAl³⁺21.0mgP92 national18%
- AmmoniumNH₄⁺15.8mgP97 national7%
- FeTotalFe20.7mgP95 national6%
- Magnesiummild; a laxative if you drink itMg²⁺8.8mgP60 national5%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺10.4mgP2 national3%
- PotassiumK⁺3.6mgP30 national0.7%
- ManganeseMn²⁺0.50mgP64 national0.1%
- ZincZn²⁺0.09mgP70 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻590mgP89 national91%
- HSO4HSO4102mgP69 national8%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5.7mgP8 national1%
- IodideI⁻2.4mgP89 national0.1%
- FluorideF⁻0.20mgP17 national0.1%
- H2PO4H₂PO₄⁻0.50mgP89 national0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃180mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.7mgP16 national
- H2SO4FreeH2SO4Free1.3mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂101mgP79 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S7.3mgP90 national
- ArsenicAs< —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