A hot (83 C at the source) blended spring drawn from four Yunomine wellheads — Ojiyu, Heisei No. 1, Heisei No. 2 and Yakata-no-yu — pooled in the 'C junction tank' before delivery. Sodium-led bicarbonate water with a chloride sub-type — the classic soda-water 'jusosen' style, soft enough to sit in the Japanese 'beauty-water' family. A generous 158 mg/kg of metasilicic acid leaves a fine silica film for noticeably smoother skin, and the wellhead carries a faint rotten-egg note of hydrogen sulfide.
Good for
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
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
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
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⁺421mgP71 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.0mgP31 national4%
- Potassiumminor; matters for drinking limitsK⁺25.5mgP70 national3%
- MagnesiumMg²⁺1.6mgP30 national0.7%
- ManganeseMn²⁺0.20mgP45 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻836mgP86 national58%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻202mgP54 national28%
- Fluoridetrace; drinking limits applyF⁻8.5mgP88 national2%
- Hydrogen sulfideHS⁻1.3mgP72 national0.2%
- ThiosulfateS₂O₃²⁻1.3mgP77 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1.3mgP8 national
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃158mgP71 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂13.3mgP63 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂145mgP85 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.3mgP79 national
- CopperCu0.01mgP76 national
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
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.
- 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.