A sodium-led bicarbonate-chloride spring from Shinhotaka in the Okuhida hot-spring district of Gifu, rising at a scalding 97 °C — among Japan's hottest natural sources, and necessarily cooled before it reaches a bath. The bicarbonate side gives it the soft juso-sen character, with a salty edge layered in from the chloride; at neutral pH and roughly 1,061 mg/kg of dissolved solids it is moderately mineralized and gentle on the skin. A generous 205 mg/kg of metasilicic acid places it squarely in Japan's beauty-water family. At the wellhead the water is clouded a clay-like greyish white and carries a faint sulfur note.
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, netto 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⁺225mgP56 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺27.9mgP44 national12%
- Potassiumminor; matters for drinking limitsK⁺31.0mgP76 national7%
- MagnesiumMg²⁺0.50mgP20 national0.3%
- AluminumAl³⁺0.10mgP56 national0.1%
- Iron (II)Fe²⁺0.10mgP29 national0.0%
- ManganeseMn²⁺0.10mgP37 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻365mgP69 national52%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻170mgP54 national42%
- Fluoridetrace; drinking limits applyF⁻6.6mgP82 national3%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻13.2mgP26 national2%
- Hydrogen sulfideHS⁻0.10mgP25 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃205mgP87 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂15.9mgP69 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂11.7mgP42 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.0010mgP6 national
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.