A sodium chloride spring rising in the Kannawa district of Beppu and running very hot at 93.0 °C — among Japan's hottest natural sources. It is weakly acidic, which gives the water a faintly tightening feel on the skin; the water itself is colorless and clear, lightly salty on the tongue with a faint metallic note. The chemical standout is metasilicic acid at 584 mg/kg, an unusually generous figure that places this water in Japan's beauty-water family and leaves skin noticeably smoother. Overall mineralization is moderate at 4,670 mg/kg of dissolved solids, enough for real body without heaviness, and the certificate's sample was drawn at the storage-tank outlet rather than at the wellhead.
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⁺1,270mgP88 national84%
- Potassiumminor; matters for drinking limitsK⁺193mgP97 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.2mgP61 national5%
- LithiumLi⁺8.2mgP97 national2%
- MagnesiumMg²⁺6.8mgP57 national0.8%
- HydrogenH⁺0.40mgP63 national0.6%
- AmmoniumNH₄⁺1.7mgP77 national0.1%
- ManganeseMn²⁺1.6mgP88 national0.1%
- Iron (II)Fe²⁺1.5mgP68 national0.1%
- StrontiumSr²⁺0.60mgP72 national0.0%
- ZincZn²⁺0.30mgP86 national0.0%
- BariumBa²⁺0.20mgP74 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,060mgP87 national88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻381mgP80 national12%
- FluorideF⁻2.8mgP64 national0.2%
- BromideBr⁻5.6mgP82 national0.1%
- HSO4HSO45.1mgP54 national0.1%
- H2PO4H₂PO₄⁻0.50mgP89 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃584mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂82.5mgP94 national
- Metaarsenious acidHAsO₂1.9mgP96 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.0mgP33 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.3mgP96 national
- LeadPb0.04mgP80 national
- CadmiumCd0.0020mgP74 national
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- Radonfaint radon — studied for joint pain (hormesis)RnNDmg
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.