A sodium chloride spring (legacy name 'junshokuen-sen') rising in the Kannawa district of Beppu. The water is salty, heat-retaining, and weakly acidic, with a slightly salty taste. The source emerges at a scalding 96 °C, placing it among Japan's hottest natural sources. It also carries a generous 193 mg/kg of metasilicic acid, leaving the kind of 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⁺1,135mgP86 national89%
- Potassiumminor; matters for drinking limitsK⁺164mgP95 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺28.8mgP45 national3%
- MagnesiumMg²⁺2.7mgP37 national0.4%
- ManganeseMn²⁺0.30mgP53 national0.1%
- Iron (II)Fe²⁺0.40mgP48 national0.0%
- Iron (III)Fe³⁺0.30mgP72 national0.0%
- AluminumAl³⁺0.10mgP54 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,721mgP85 national87%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻346mgP77 national13%
- BicarbonateHCO₃⁻12.2mgP8 national0.4%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃193mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂74.5mgP93 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂22.0mgP53 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 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.