The Shirahata source at Kusatsu — a strongly acidic, sulfur-bearing aluminium sulfate-chloride spring of the hydrogen sulfide type. At pH 2.0 it is sharp enough to sting slightly on contact; the water runs colorless and clear at the wellhead yet carries an unmistakable rotten-egg note of hydrogen sulfide. Hydrogen ions alone make up more than a third of the cation balance, which is where the bite comes from, while an unusually generous 240.6 mg/kg of metasilicic acid leaves a thin silica film that takes some of the edge off. Dissolved solids of 1,780 mg/kg put it in moderately mineralized territory, with sulfate and chloride providing the body.
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⁺9.3mgP87 national35%
- Aluminumshows up in acidic volcanic waterAl³⁺47.1mgP95 national20%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺78.0mgP66 national15%
- Magnesiummild; a laxative if you drink itMg²⁺41.2mgP84 national13%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺67.0mgP20 national11%
- Potassiumminor; matters for drinking limitsK⁺26.3mgP72 national3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺18.5mgP94 national3%
- ManganeseMn²⁺1.9mgP89 national0.3%
- AmmoniumNH₄⁺0.30mgP39 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻686mgP92 national54%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻328mgP67 national35%
- HSO4HSO4216mgP82 national8%
- Fluoridetrace; drinking limits applyF⁻10.0mgP91 national2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃241mgP94 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.6mgP53 national
- H2SO4FreeH2SO4Free5.1mgP83 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂29.3mgP59 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S6.4mgP87 national
- ArsenicAs0.25mgP84 national
- LeadPb0.01mgP56 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