A blended source pooled in storage tank #1 at Shuzenji on the Izu Peninsula, classified as an alkaline simple thermal spring. At pH 8.51 the water has the slippery, soft-on-the-skin feel of the Japanese 'beauty-water' family. The wellhead runs hot (60.1 °C) — among the warmer Shuzenji sources — and a modest 63 mg/kg of metasilicic acid adds a touch of silky finish.
Good for
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
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
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⁺146mgP39 national88%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺13.4mgP29 national9%
- Potassiumminor; matters for drinking limitsK⁺5.9mgP36 national2%
- MagnesiumMg²⁺0.50mgP20 national0.6%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻133mgP46 national51%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻118mgP56 national33%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻52.2mgP30 national12%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻8.6mgP45 national4%
- Bromidetrace, sea-originBr⁻0.20mgP29 national
- Fluoridetrace; drinking limits applyF⁻0.20mgP17 national
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national
- HydroxideOH⁻0.07mgP35 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃63.2mgP37 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.4mgP38 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.