A strongly acidic sulfate spring (pH 1.9-2.0) loaded with iron and aluminum — a rare chemistry profile in Japan. The water emerges at over 50 °C carrying a faint yellow tint, oxidizing reddish-brown on contact with air, with sharp prickling acidity and a clear metallic-astringent taste. Metasilicic acid at a generous 292 mg/kg adds a thin silica film on top of the iron-rich body. The 'kakou-no-izumi' (crater spring) has a long track record for stubborn skin conditions.
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
From a certificate analyzed in 2024.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were removed from the official list of contraindications then, or include a condition that was. If you have a health condition, ask a doctor before bathing.
- Acute illness (especially when fever is present)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Bleeding disorders
- Severe anemia
- Respiratory failure
- Renal failure
- People with sensitive skin or mucous membranes, or with photosensitivity symptoms (includes a condition dropped from the official list in 2014)
- Any illness in an actively progressing stage
- Pregnancy (especially the early and late stages) (dropped from the official list in 2014)
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⁺240mgP100 national54%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺10.1mgP92 national20%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺113mgP100 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺61.9mgP61 national6%
- Iron (III)Fe³⁺45.2mgP100 national5%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺23.7mgP5 national2%
- ManganeseMn²⁺1.4mgP85 national0.1%
- ZincZn²⁺0.60mgP93 national0.0%
- MagnesiumMg²⁺—mg0.0%
- StrontiumSr²⁺0.40mgP61 national0.0%
- Potassiumminor; matters for drinking limitsK⁺—mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,850mgP100 national86%
- HSO4HSO4622mgP97 national14%
- ChlorideCl⁻2.1mgP3 national0.1%
- FluorideF⁻0.20mgP16 national0.0%
- H2PO4H₂PO₄⁻0.60mgP95 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃292mgP97 national
- H2SO4FreeH2SO4Free15.7mgP93 national
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.2mgP22 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2024.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were dropped from the official list then.
- Diabetes
- Menstrual disorders (dropped from the official list in 2014)
- Chronic digestive disorders
- Cold sensitivity
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