A simple thermal spring — gentle, lightly mineralized water with nothing sharp about it. It is weakly alkaline and soft on the skin, and at 0.59 g/kg of dissolved solids it sits on the light side of the mineralization scale. What lifts it out of the ordinary is the silica: 172 mg/kg of metasilicic acid puts this water squarely in Japan's beauty-water family, leaving the kind of thin, moisturizing film that makes skin feel noticeably smoother once you are out of the bath.
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⁺100mgP28 national73%
- Potassiumminor; matters for drinking limitsK⁺24.7mgP69 national11%
- Magnesiummild; a laxative if you drink itMg²⁺6.2mgP55 national9%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.5mgP24 national8%
- LithiumLi⁺0.20mgP38 national0.5%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻76.9mgP33 national35%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻109mgP44 national29%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻53.2mgP40 national18%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻33.0mgP86 national18%
- FluorideF⁻0.30mgP22 national0.3%
- Hydrogen phosphateHPO₄²⁻0.60mgP74 national0.2%
- BromideBr⁻0.20mgP29 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃172mgP77 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.8mgP28 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.05mgP52 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.