A hot sodium-calcium chloride spring drawn from a tank that blends four Kinosaki wells (No. 27, 28, 29, and 30). The salty, heat-retaining chloride water is moderately mineralized with noticeable body. With 86.7 mg/kg of metasilicic acid it leaves a thin silica film that makes skin feel smoother.
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⁺1,030mgP85 national65%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺457mgP92 national33%
- Potassiumminor; matters for drinking limitsK⁺47.9mgP84 national2%
- MagnesiumMg²⁺2.0mgP35 national0.2%
- StrontiumSr²⁺5.5mgP91 national0.2%
- LithiumLi⁺0.80mgP70 national0.2%
- BariumBa²⁺1.1mgP90 national0.0%
- ManganeseMn²⁺0.30mgP53 national0.0%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.04mgP17 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.02mgP22 national
- ZincZn²⁺0.01mgP41 national
- CopperCu²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,300mgP88 national98%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻53.4mgP29 national1%
- FluorideF⁻2.4mgP62 national0.2%
- BromideBr⁻8.0mgP87 national0.1%
- IodideI⁻0.20mgP52 national0.0%
- NitrateNO₃⁻0.20mgP36 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃86.7mgP49 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.5mgP34 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂9.4mgP40 national
- ArsenicAs0.07mgP59 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0005mgP51 national
- 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.