The Jizo source at Kusatsu yields sharply acidic water: at pH 2.0 it stings slightly on contact, built on aluminium with sulfate and chloride behind it, and carrying enough hydrogen sulfide to be classed as a sulfur spring of the H₂S type. It runs colorless and clear, yet announces itself with an unmistakable rotten-egg note. Mineralization is moderate at roughly 1,750 mg/kg of dissolved solids, but metasilicic acid is generous at 234 mg/kg, leaving a thin silica film that softens the edge of the acidic bite.
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
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.
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺9.6mgP88 national37%
- Aluminumshows up in acidic volcanic waterAl³⁺46.2mgP95 national20%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺73.6mgP64 national14%
- Magnesiummild; a laxative if you drink itMg²⁺39.6mgP82 national13%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺62.9mgP18 national11%
- Potassiumminor; matters for drinking limitsK⁺25.7mgP71 national3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺18.3mgP94 national3%
- ManganeseMn²⁺1.9mgP89 national0.3%
- AmmoniumNH₄⁺0.30mgP39 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻679mgP92 national55%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻314mgP66 national34%
- HSO4HSO4218mgP83 national9%
- Fluoridetrace; drinking limits applyF⁻9.6mgP90 national2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃234mgP93 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.8mgP54 national
- H2SO4FreeH2SO4Free5.3mgP84 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂57.2mgP71 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S6.9mgP88 national
- ArsenicAs0.23mgP82 national
- LeadPb0.01mgP49 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
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