The Yubatake source at Kusatsu — sharply acidic water at pH 2.0 that also qualifies as a sulfur spring, carrying the unmistakable rotten-egg note of hydrogen sulfide right at the wellhead. The certificate classifies it as a hypotonic, acidic, high-temperature spring of the aluminium–sulfate/chloride type, and at this acidity the water prickles noticeably on the skin. Mineralization is moderate at 1,780 mg/kg of dissolved solids, but metasilicic acid is unusually generous at 243 mg/kg, leaving the kind of thin silica film that makes skin feel smoother afterwards.
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.
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺10.0mgP90 national38%
- Aluminumshows up in acidic volcanic waterAl³⁺49.1mgP95 national21%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺73.1mgP64 national14%
- Magnesiummild; a laxative if you drink itMg²⁺39.4mgP82 national12%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺59.7mgP17 national10%
- Potassiumminor; matters for drinking limitsK⁺30.0mgP74 national3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺17.8mgP94 national2%
- ManganeseMn²⁺2.0mgP90 national0.3%
- AmmoniumNH₄⁺0.50mgP49 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻674mgP92 national54%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻329mgP67 national35%
- HSO4HSO4227mgP85 national9%
- Fluoridetrace; drinking limits applyF⁻10.1mgP92 national2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃243mgP94 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.8mgP55 national
- H2SO4FreeH2SO4Free5.7mgP87 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂46.9mgP68 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S6.2mgP87 national
- ArsenicAs0.30mgP88 national
- LeadPb0.01mgP54 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