An alkaline simple thermal spring drawn at a hot 51.7 °C with a markedly alkaline pH of 9.1. The water is hypotonic and gentle on the body, with the slippery beauty-water feel that strong alkalinity gives. Metasilicic acid is generous at 121 mg/kg, leaving the kind of silica film that makes skin feel noticeably smoother afterward. Sourced from the Hakone Ohiradai cooperative blend (Onsenmura wells No. 87, 91, 115) and cooled to a 43–44 °C soaking range.
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⁺255mgP57 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺23.5mgP42 national
- Potassiumminor; matters for drinking limitsK⁺15.5mgP54 national
- Magnesiummild; a laxative if you drink itMg²⁺1.7mgP31 national
- LithiumLi⁺0.28mgP40 national
- StrontiumSr²⁺0.10mgP26 national
- Manganesetrace; companion to iron springsMn²⁺0.04mgP20 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.01mgP19 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.01mgP12 national
- AmmoniumNH₄⁺0.0000mg
- ZincZn²⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻352mgP69 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻56.3mgP30 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻35.2mgP36 national
- HSiO3HSiO₃⁻30.1mgP69 national
- BO2BO₂⁻8.6mgP88 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻5.4mgP36 national
- Bromidetrace, sea-originBr⁻2.2mgP71 national
- NitrateNO₃⁻0.84mgP70 national
- Fluoridetrace; drinking limits applyF⁻0.40mgP31 national
- HydroxideOH⁻0.21mgP67 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃121mgP62 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.1mgP59 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.07mgP10 national
- ArsenicAs0.31mgP88 national
- CadmiumCd0.0000mg
- CopperCu0.0000mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.0000mg
- LeadPb0.0000mg
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.