A gentle simple thermal spring — lightly mineralized and easy on the body, with no single ion strong enough to give it an edge. It is weakly alkaline and soft on the skin, which puts it squarely in Japan's beauty-water family. What sets it apart is metasilicic acid at 243 mg/kg: silica that generous leaves a thin film on the skin and a noticeably smoother feel afterwards. The wellhead runs hot enough to sit among Japan's hottest natural sources, while the water itself is colorless, clear, and all but tasteless and odorless.
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⁺153mgP37 national84%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.9mgP30 national7%
- Potassiumminor; matters for drinking limitsK⁺13.5mgP52 national4%
- Magnesiummild; a laxative if you drink itMg²⁺3.2mgP40 national3%
- LithiumLi⁺0.20mgP39 national0.4%
- ManganeseMn²⁺0.10mgP39 national0.0%
- StrontiumSr²⁺0.10mgP25 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻275mgP61 national55%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻78.2mgP35 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻69.3mgP49 national18%
- FluorideF⁻0.20mgP19 national0.1%
- BromideBr⁻0.20mgP31 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃243mgP94 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.4mgP33 national
- Metaarsenious acidHAsO₂0.10mgP59 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.09mgP71 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.