A sodium chloride spring drawn from the Kinryu Jigoku source in Beppu's Kannawa district. At 86.5 °C it ranks among Japan's hottest natural sources, weakly acidic and salty with strong heat-retention. It is unusually rich in metasilicic acid at 566 mg/kg, leaving a thin silica film that makes skin feel noticeably 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⁺1,200mgP87 national86%
- Potassiumminor; matters for drinking limitsK⁺181mgP96 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺39.8mgP52 national3%
- LithiumLi⁺8.5mgP98 national2%
- MagnesiumMg²⁺4.2mgP44 national0.6%
- AmmoniumNH₄⁺1.3mgP70 national0.1%
- ManganeseMn²⁺1.1mgP78 national0.1%
- FeTotalFe1.4mgP69 national
- StrontiumSr²⁺0.50mgP68 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.30mgP43 national
- BariumBa²⁺0.20mgP74 national
- CopperCu²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,910mgP86 national88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻338mgP77 national12%
- FluorideF⁻3.0mgP66 national0.3%
- BromideBr⁻5.6mgP82 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃566mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂91.4mgP95 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.8mgP52 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.