An alkaline simple thermal spring in the Gero Onsen style. With a pH around 9 it sits in the Japanese beauty-water family, giving the skin a soft, slippery feel. The water is only lightly mineralized and gentle on the body, while close to 59 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother afterwards.
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⁺49.8mgP13 national88%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.0mgP11 national6%
- Potassiumminor; matters for drinking limitsK⁺1.9mgP22 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.1mgP21 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.7mgP25 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻12.0mgP56 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5.0mgP7 national
- NitrateNO₃⁻0.70mgP67 national
- Fluoridetrace; drinking limits applyF⁻0.50mgP32 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃58.9mgP35 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP43 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.09mgP11 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.