A sodium-led chloride spring with bicarbonate softness layered in. It is alkaline (around pH 8.5), giving the slippery beauty-water feel on the skin, and emerges hot at about 41 °C. Moderate mineralization with chloride and bicarbonate as the defining ions makes for a salty, heat-retaining soak.
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⁺558mgP77 national95%
- Potassiumminor; matters for drinking limitsK⁺12.5mgP49 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.0mgP11 national
- LithiumLi⁺2.2mgP92 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻595mgP76 national66%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻435mgP73 national28%
- Fluoridetrace; drinking limits applyF⁻16.3mgP97 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻1.7mgP78 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.7mgP20 national
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.