Kusatsu's Bandaiko source — a 94.6 °C top-decile hot spring with pH 1.7, among Japan's most strongly acidic waters. A hydrogen-led acidic sulfate-chloride spring, it carries the sharp, prickly bite that defines the kusatsu style and has a long track record on stubborn skin conditions such as atopic dermatitis. With 26.4 mg/kg of free hydrogen sulfide layered on top, it also carries the unmistakable rotten-egg note of H₂S.
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
- 皮膚または粘膜の過敏な方
- 光線過敏症の方
- 病気の活動期(特に熱のあるとき)
- Active tuberculosis
- 進行した悪性腫瘍
- 高度の貧血など身体衰弱の著しい場合
- 少し動くと息切れがするような重い心臓または肺の病気
- むくみのあるような腎臓の病気
- 消化管出血
- 目に見える出血があるとき
- 慢性の病気の活動期
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⁺138mgP33 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺114mgP73 national
- Magnesiummild; a laxative if you drink itMg²⁺76.0mgP88 national
- Potassiumminor; matters for drinking limitsK⁺64.4mgP86 national
- Aluminumshows up in acidic volcanic waterAl³⁺43.2mgP100 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺19.9mgP100 national
- Manganesetrace; companion to iron springsMn²⁺3.6mgP96 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺3.0mgP79 national
- HSO4HSO4867mgP100 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻860mgP95 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻708mgP75 national
- Fluoridetrace; drinking limits applyF⁻25.1mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃528mgP99 national
- H2SO4FreeH2SO4Free28.5mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.4mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP57 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S26.4mgP97 national
Measured at the source
On the certificate
- 糖尿病性皮膚搔痒症
- 高齢者の皮膚搔痒症
- 打撲
- Chronic digestive disorders
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