A sodium chloride spring of the weakly alkaline, heat-retaining family. The source runs hot at 70 °C — among Japan's hotter natural sources — yet the water feels soft on the skin. Moderate mineralization at 1,210 mg/kg dissolved solids gives it noticeable body without being heavy.
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, netto 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⁺346mgP67 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.2mgP53 national
- Potassiumminor; matters for drinking limitsK⁺8.0mgP41 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.11mgP54 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.01mgP19 national
- BariumBa²⁺0.0050mgP27 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻508mgP76 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻81.2mgP40 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻71.4mgP49 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻3.0mgP26 national
- Fluoridetrace; drinking limits applyF⁻0.57mgP30 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃124mgP63 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂22.7mgP76 national
- ArsenicAs< —mg
- CopperCu< —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.
- 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.