An alkaline simple thermal spring drawing on a blend of the Yumoto No. 37, 50, and 110 wells at Tonosawa in Hakone. At pH 9.1 it is strongly alkaline, giving the slippery, skin-smoothing feel of Japan's beauty-water family. The water is colorless, transparent, and odorless, gentle on the body with little mineral edge.
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⁺131mgP36 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺22.1mgP41 national
- Potassiumminor; matters for drinking limitsK⁺1.3mgP15 national
- Magnesiummild; a laxative if you drink itMg²⁺0.06mgP6 national
- StrontiumSr²⁺0.02mgP12 national
- LithiumLi⁺0.01mgP11 national
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- ZincZn²⁺NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻128mgP59 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻124mgP44 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻29.3mgP17 national
- HSiO3HSiO₃⁻10.5mgP60 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻2.8mgP27 national
- BO2BO₂⁻2.8mgP72 national
- Bromidetrace, sea-originBr⁻0.26mgP31 national
- Fluoridetrace; drinking limits applyF⁻0.23mgP18 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃42.5mgP23 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.6mgP27 national
- ArsenicAs0.16mgP76 national
- CadmiumCdNDmg
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.