A sodium-led chloride spring with bicarbonate softness layered in, surfacing hot at 50.1 °C (historically classed as a soda-salt spring). It is neutral in pH, slightly salty with a faint mineral note, and carries a very faint yellow tint at the source. A generous 157 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel 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, 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⁺1,550mgP90 national91%
- Potassiumminor; matters for drinking limitsK⁺114mgP93 national4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺55.2mgP58 national4%
- Magnesiummild; a laxative if you drink itMg²⁺9.7mgP61 national1%
- AmmoniumNH₄⁺2.2mgP83 national0.2%
- LithiumLi⁺0.50mgP54 national0.1%
- BariumBa²⁺0.70mgP84 national0.0%
- Iron (II)Fe²⁺0.60mgP56 national0.0%
- StrontiumSr²⁺0.60mgP74 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,220mgP88 national79%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻982mgP90 national20%
- BromideBr⁻5.1mgP82 national0.1%
- SulfateSO₄²⁻1.7mgP9 national0.1%
- FluorideF⁻0.80mgP43 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃157mgP71 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂16.7mgP70 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.0mgP34 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.