An exceptionally hot 88 C source in Beppu's Kannawa quarter — a sodium chloride spring with bicarbonate softness layered in. The water reads slightly acidic at the wellhead (pH around 3.0), giving the salt-led mineralization a faint prickle, and sits among Japan's hottest natural sources.
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⁺748mgP82 national81%
- Magnesiummild; a laxative if you drink itMg²⁺40.5mgP83 national8%
- Potassiumminor; matters for drinking limitsK⁺96.9mgP92 national6%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺24.4mgP42 national3%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺1.0mgP66 national2%
- AmmoniumNH₄⁺6.7mgP94 national0.9%
- Aluminumshows up in acidic volcanic waterAl³⁺0.40mgP74 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,000mgP80 national73%
- FluorideF⁻2.6mgP64 national0.3%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻473mgP75 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻16.5mgP28 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃275mgP96 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂39.7mgP84 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.4mgP29 national
- CadmiumCd< —mg
- CopperCu< —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.