A simple thermal spring drawn from beneath Isawa in Fuefuki, lightly mineralized at 438.6 mg/kg of dissolved solids. Weakly alkaline and soft on the skin, it carries a fair 55.4 mg/kg of metasilicic acid, leaving a thin silica film for a smoother feel. The water is colorless and clear, with a faint salty tang and a light hydrogen sulfide note at the source.
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⁺112mgP33 national89%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.5mgP23 national8%
- Magnesiummild; a laxative if you drink itMg²⁺1.1mgP27 national2%
- Potassiumminor; matters for drinking limitsK⁺2.9mgP26 national1%
- AmmoniumNH₄⁺1.1mgP67 national1%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻117mgP43 national57%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻90.4mgP40 national26%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻47.4mgP39 national17%
- FluorideF⁻0.20mgP17 national0.2%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃55.4mgP33 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.6mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- autonomic nervous system instability, insomnia, depressive states
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.