A sodium-led bicarbonate–chloride spring — the classic 'jūsō-sen' style with a salt-tinged finish from a chloride sub-type. At the wellhead it carries a faint rotten-egg note of H₂S and reads salty on the tongue. Hot (64.7 °C) and neutral, with 5.7 g/kg of dissolved solids — soft on the skin but holds warmth well.
Good for
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
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
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
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⁺1,586mgP90 national90%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺60.5mgP60 national4%
- Potassiumminor; matters for drinking limitsK⁺47.2mgP83 national3%
- Magnesiummild; a laxative if you drink itMg²⁺24.4mgP73 national3%
- ManganeseMn²⁺0.70mgP72 national0.0%
- FeTotalFe0.60mgP59 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,434mgP98 national51%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,314mgP83 national48%
- Fluoridetrace; drinking limits applyF⁻4.9mgP78 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1.0mgP7 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂172mgP98 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃41.6mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂82.5mgP76 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.