Water drawn from Kusatsu Onsen's Bandaiko source — a fiercely acidic spring at pH 1.7, classified as an acidic hydrogen-sulfate and chloride spring, where the sharp prickle of the acid is backed by quiet sulfate body and a salty chloride edge. The posted analysis lists atopic dermatitis, psoriasis vulgaris and pyoderma among its spring-type indications, the classic profile of a strongly bactericidal acidic water. At 94.6 °C the source ranks among Japan's hottest, yet the panel records it arriving in the open-air baths at 42 °C with no dilution, no reheating, no recirculation and no disinfection — untreated free-flow. A generous 528 mg/kg of metasilicic acid leaves a thin silica film behind, softening what is otherwise a bracing soak.
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, netto 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
- People with sensitive skin or mucous membranes
- Dry skin in the elderly
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Marked physical debilitation, such as advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Visible bleeding
- Acute exacerbation of a chronic illness
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺138mgP37 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺114mgP76 national
- Magnesiummild; a laxative if you drink itMg²⁺76.0mgP89 national
- Potassiumminor; matters for drinking limitsK⁺64.4mgP88 national
- Aluminumshows up in acidic volcanic waterAl³⁺43.2mgP94 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺19.9mgP97 national
- Manganesetrace; companion to iron springsMn²⁺3.6mgP95 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺3.0mgP77 national
- AmmoniumNH₄⁺0.40mgP43 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻840mgP94 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻708mgP78 national
- HSO4HSO4565mgP95 national
- Fluoridetrace; drinking limits applyF⁻25.1mgP99 national
- Bromidetrace, sea-originBr⁻4.5mgP80 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃528mgP99 national
- H2SO4FreeH2SO4Free28.5mgP96 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.4mgP75 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP58 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs2.6mgP99 national
- LeadPb0.08mgP93 national
- CopperCu0.0000mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.0000mg
Measured at the source
On the certificate
- Impaired glucose tolerance (diabetes)
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Stress-related symptoms (sleep disorders, depressed mood)
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