A sodium chloride spring drawn from Beppu City's municipal hot-spring supply network (Kannawa line) and sampled from the storage tank that feeds the distribution system. At 81.7 °C at the source it ranks among Japan's hottest natural waters, and it comes through weakly acidic with a distinctly salty tang on the lips. Dissolved solids of 4,487 mg/kg give it noticeable body, and the chloride chemistry is the heat-retaining kind that keeps you warm long after you climb out. The standout figure is metasilicic acid at 685 mg/kg, an unusually generous load that leaves a fine silica film and places this water squarely in Japan's beauty-water family.
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⁺1,190mgP87 national87%
- Potassiumminor; matters for drinking limitsK⁺178mgP96 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.5mgP51 national3%
- LithiumLi⁺7.8mgP97 national2%
- MagnesiumMg²⁺4.3mgP45 national0.6%
- AmmoniumNH₄⁺1.0mgP66 national0.1%
- ManganeseMn²⁺1.2mgP80 national0.1%
- StrontiumSr²⁺0.50mgP68 national0.0%
- Iron (II)Fe²⁺0.40mgP48 national0.0%
- BariumBa²⁺0.20mgP74 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,950mgP86 national88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻334mgP77 national11%
- FluorideF⁻2.9mgP65 national0.2%
- BromideBr⁻5.4mgP82 national0.1%
- HSO4HSO40.90mgP47 national0.0%
- IodideI⁻0.20mgP52 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃685mgP100 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂84.7mgP95 national
- Metaarsenious acidHAsO₂1.9mgP96 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂15.0mgP48 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.3mgP96 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- 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.