A self-flowing source in Beppu's Myoban district, classified as a strongly acidic simple sulfur spring (hydrogen-sulfide type) at pH 2.7. At the wellhead the water is colorless and clear yet carries a sharp metallic edge and a strong rotten-egg note of H₂S. It is also rich in metasilicic acid (145 mg/kg), the kind of silica that leaves a thin film for noticeably smoother skin.
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⁺2.0mgP71 national49%
- Aluminumshows up in acidic volcanic waterAl³⁺5.0mgP84 national14%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺12.2mgP3 national13%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺10.3mgP27 national13%
- Magnesiummild; a laxative if you drink itMg²⁺3.8mgP43 national8%
- Potassiumminor; matters for drinking limitsK⁺4.7mgP33 national3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.8mgP70 national1%
- StrontiumSr²⁺5.0mgP89 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻191mgP64 national95%
- HSO4HSO412.9mgP59 national3%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2.6mgP3 national2%
- ThiosulfateS₂O₃²⁻0.40mgP62 national0.2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃145mgP67 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂—mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂74.8mgP75 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S14.1mgP96 national
- ArsenicAs< —mg
- CadmiumCd< —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