A sodium-chloride spring from the Ninotaira source. The wellhead runs at 84 °C — among Japan's hottest natural sources — and is diluted with water to bring the bath down to 40–42 °C, with the salt content carrying long-lasting warmth. Weakly alkaline (pH 8.3) and soft on the skin, the water is unusually rich in metasilicic acid at 272 mg/kg, leaving the kind of thin silica film that makes skin feel noticeably smoother afterwards.
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⁺988mgP84 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺80.4mgP67 national
- Potassiumminor; matters for drinking limitsK⁺63.3mgP87 national
- Magnesiummild; a laxative if you drink itMg²⁺1.4mgP29 national
- LithiumLi⁺0.58mgP56 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.11mgP54 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.01mgP13 national
- Manganesetrace; companion to iron springsMn²⁺0.0050mgP15 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0010mgP49 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,673mgP85 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻75.7mgP50 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻73.7mgP36 national
- Fluoridetrace; drinking limits applyF⁻0.12mgP10 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃272mgP96 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂82.1mgP94 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0005mgP10 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.