A hot (81.7 °C) sodium chloride spring drawn from Beppu's Kannawa-Shibu district. Weakly acidic with a moderately salty edge, and rich in metasilicic acid at 685 mg/kg — the kind of silica film that leaves skin noticeably smoother. The water is held in a storage tank to cool from one of Japan's hottest source temperatures before reaching the bath.
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⁺1,188mgP87 national87%
- Potassiumminor; matters for drinking limitsK⁺178mgP96 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.1mgP51 national3%
- LithiumLi⁺8.1mgP97 national2%
- MagnesiumMg²⁺4.3mgP45 national0.6%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,950mgP86 national88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻334mgP77 national11%
- BromideBr⁻5.4mgP82 national0.2%
- HSO4HSO4—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃685mgP100 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂86.4mgP95 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.5mgP29 national
- ArsenicAs15.0mgP100 national
- LeadPb1.3mgP100 national
- CadmiumCd< —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.