十王堂の湯Juodo-no-yu Spring
A simple sulfur spring rising in Nozawa Onsen village, Nagano. It surfaces at 71.9 °C — among Japan's hotter natural sources — colorless and clear at the spout, but carrying a strong hydrogen sulfide note and a sulfurous taste. Dissolved solids are light at 945 mg/kg, yet metasilicic acid runs to 110 mg/kg; paired with a mildly alkaline pH, that silica leaves the soft, smoothing finish that places the water in Japan's beauty-water family.
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
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺189mgP49 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.0mgP61 national27%
- Potassiumminor; matters for drinking limitsK⁺5.1mgP34 national1%
- AmmoniumNH₄⁺1.3mgP71 national0.6%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻414mgP83 national69%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻82.6mgP36 national19%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻45.8mgP28 national6%
- Hydrogen sulfidesulfur character — antibacterialHS⁻16.6mgP97 national4%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.0mgP46 national2%
- FluorideF⁻0.90mgP43 national0.4%
- Bromidetrace, sea-originBr⁻0.30mgP37 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃110mgP59 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.6mgP48 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.7mgP25 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.90mgP74 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.
- 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