A blended simple thermal water drawn from three Ito Onsen wells (Oka No. 173, Kamata No. 41, Oka No. 277) and mixed in an underground storage tank before being sent to the baths. Total dissolved solids sit at 696 mg/kg with a pH of 8.2 — weakly alkaline, gentle on the skin, and squarely in simple-spring territory. The wellhead is hot at 51 degrees Celsius and the water is cooled to the bathing range of 39 to 42 degrees before serving.
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⁺144mgP39 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.0mgP61 national
- Potassiumminor; matters for drinking limitsK⁺2.9mgP26 national
- StrontiumSr²⁺0.30mgP55 national
- Magnesiummild; a laxative if you drink itMg²⁺0.10mgP11 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻340mgP77 national
- Fluoridetrace; drinking limits applyF⁻70.5mgP100 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻22.9mgP14 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.5mgP44 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻0.10mgP0 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃29.5mgP12 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.3mgP21 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂13.2mgP46 national
- ArsenicAs0.06mgP57 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.