A steam-generated acidic spring fed by the volcanic fumaroles of Owakudani, leaning to sulfate and chloride. At pH 2.1 it is strongly acidic, sharp enough to prickle slightly on the skin, and arrives slightly cloudy-white with a faint hydrogen sulfide note. A generous 157 mg/kg of metasilicic acid leaves a thin silica film that smooths the 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
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺90.6mgP70 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺40.5mgP10 national
- Magnesiummild; a laxative if you drink itMg²⁺34.7mgP80 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺13.2mgP91 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺8.0mgP85 national
- Aluminumshows up in acidic volcanic waterAl³⁺7.5mgP89 national
- Potassiumminor; matters for drinking limitsK⁺3.2mgP27 national
- Manganesetrace; companion to iron springsMn²⁺1.3mgP82 national
- AmmoniumNH₄⁺0.22mgP33 national
- StrontiumSr²⁺0.17mgP29 national
- ZincZn²⁺0.04mgP66 national
- LithiumLi⁺0.0000mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻443mgP84 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻342mgP68 national
- HSO4HSO4118mgP78 national
- Fluoridetrace; drinking limits applyF⁻0.80mgP44 national
- NitrateNO₃⁻0.49mgP57 national
- Bromidetrace, sea-originBr⁻0.14mgP17 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃157mgP71 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.2mgP66 national
- H2SO4FreeH2SO4Free2.4mgP82 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.3mgP78 national
- ArsenicAs0.03mgP43 national
- LeadPb0.02mgP76 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0010mgP79 national
- CadmiumCd0.0000mg
- CopperCu0.0000mg
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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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