A strongly acidic, sulfur-bearing spring from the Myoban district of Beppu, certified as an acidic sulfur-containing simple thermal water at pH 2.3 — sharp enough to prickle slightly on the skin. At the source it runs colorless but faintly clouded, carrying an unmistakable rotten-egg note of H₂S and a weakly astringent taste on the lips. Dissolved solids are light at 772 mg/kg, yet metasilicic acid reaches 160.8 mg/kg, leaving the kind of thin silica film that makes skin feel noticeably 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
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⁺5.0mgP78 national60%
- Aluminumshows up in acidic volcanic waterAl³⁺14.0mgP90 national19%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺13.2mgP3 national7%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.3mgP28 national7%
- Magnesiummild; a laxative if you drink itMg²⁺3.7mgP42 national4%
- Potassiumminor; matters for drinking limitsK⁺6.9mgP39 national2%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺4.0mgP78 national2%
- AmmoniumNH₄⁺0.20mgP30 national0.1%
- ManganeseMn²⁺0.10mgP36 national0.0%
- StrontiumSr²⁺0.10mgP26 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻468mgP86 national91%
- HSO4HSO478.2mgP65 national8%
- ChlorideCl⁻3.0mgP4 national0.8%
- ThiosulfateS₂O₃²⁻1.6mgP79 national0.3%
- H2PO4H₂PO₄⁻0.60mgP94 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃161mgP74 national
- H2SO4FreeH2SO4Free1.0mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂86.2mgP76 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S5.6mgP87 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0005mgP51 national
- ArsenicAs< —mg
- CopperCu< —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