A sodium-calcium chloride spring served at 44.3 °C — among the hotter natural sources in the region. Weakly alkaline and soft on the skin, with 62.7 mg/kg of metasilicic acid leaving a faint silica film. The water carries a light yellow-brown tint at the source, a salty tang on the lips and a mild mineral note. Delivered straight to the tubs as untreated source flow — no dilution, heating, recirculation or disinfection.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺291mgP59 national55%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺187mgP82 national41%
- LithiumLi⁺0.51mgP55 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻877mgP78 national90%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻137mgP46 national10%
- Fluoridetrace; drinking limits applyF⁻4.5mgP77 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃62.7mgP40 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.7mgP74 national
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.