A strongly acidic spring at pH 1.7 — sharp enough to prickle on the skin — classified as an acidic, iron(II)-bearing aluminium-sulfate-chloride hot spring. Iron at 30 mg/kg is high enough to oxidize reddish-brown on contact with air, and 3,439 mg/kg of dissolved solids give the water real body, with sulfate adding quiet weight and chloride a salty edge. Metasilicic acid runs to 208.8 mg/kg, leaving the kind of thin silica film that makes skin feel noticeably smoother afterwards. The posted analysis was sampled in the open-air bath itself rather than at a wellhead, and the bath is fed by two blended wells, Kagebata and Domon.
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.
- Aluminumshows up in acidic volcanic waterAl³⁺99.9mgP98 national
- Magnesiummild; a laxative if you drink itMg²⁺80.8mgP90 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺75.9mgP66 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺53.9mgP15 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺30.0mgP96 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺22.5mgP99 national
- Potassiumminor; matters for drinking limitsK⁺20.0mgP62 national
- Manganesetrace; companion to iron springsMn²⁺4.0mgP97 national
- ZincZn²⁺0.40mgP91 national
- LithiumLi⁺0.04mgP15 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,322mgP98 national
- HSO4HSO4997mgP99 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻449mgP73 national
- Fluoridetrace; drinking limits applyF⁻10.6mgP92 national
- Bromidetrace, sea-originBr⁻1.0mgP59 national
- Iodideyellows in air; helps blood lipidsI⁻1.0mgP79 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃209mgP88 national
- H2SO4FreeH2SO4Free56.4mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.2mgP38 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂114mgP82 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- ArsenicAs0.01mgP24 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.
- 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