A sodium chloride spring at Kannawa in Beppu, rising at a scalding 96.2 °C — among Japan's hottest natural sources. The certificate records it under its older name as a pure saline spring: weakly acidic, moderately mineralized at 3,679 mg/kg of dissolved solids, with a faint salty tang on the lips and the heat-retaining body that chloride water is known for. A generous 193 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably 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⁺1,135mgP87 national89%
- Potassiumminor; matters for drinking limitsK⁺164mgP96 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺28.8mgP45 national3%
- MagnesiumMg²⁺2.7mgP38 national0.4%
- ManganeseMn²⁺0.70mgP73 national0.1%
- Iron (II)Fe²⁺0.40mgP49 national0.0%
- Iron (III)Fe³⁺0.30mgP71 national0.0%
- AluminumAl³⁺0.10mgP55 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,721mgP86 national87%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻346mgP78 national13%
- BicarbonateHCO₃⁻12.2mgP8 national0.4%
- CarbonateCO₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃193mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂74.5mgP94 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂22.0mgP52 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 national
- ArsenicAs1.9mgP98 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.