A simple thermal spring — gentle water with nothing sharp about it. It is weakly alkaline and soft on the skin, and although the total dissolved load is light at about 0.9 g/kg, the water carries a generous 221 mg/kg of metasilicic acid, which puts it squarely in Japan's beauty-water family. At the wellhead it is clear with only a very faint yellow tint, almost tasteless and odorless, and the silica leaves the kind of thin film that makes skin feel noticeably smoother afterwards.
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⁺101mgP30 national47%
- Magnesiummild; a laxative if you drink itMg²⁺29.3mgP75 national26%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺43.1mgP55 national23%
- Potassiumminor; matters for drinking limitsK⁺13.2mgP51 national4%
- LithiumLi⁺0.60mgP61 national1.0%
- Iron (II)Fe²⁺0.50mgP52 national0.2%
- ManganeseMn²⁺0.30mgP53 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- BariumBa²⁺0.10mgP53 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻348mgP68 national60%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻81.6mgP36 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻72.6mgP48 national16%
- FluorideF⁻0.10mgP9 national0.1%
- BromideBr⁻0.30mgP36 national0.0%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃221mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.7mgP28 national
- Metaarsenious acidHAsO₂0.10mgP56 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
- ArsenicAs0.07mgP59 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.