A sodium chloride spring drawn from the Oniyama Jigoku source in Beppu's Kannawa district. Neutral and hypotonic, it carries 2,279 mg/kg of dissolved solids — moderate mineralization with the salty, heat-retaining character chloride water is known for, so the warmth lingers well after the bath. Metasilicic acid at 154 mg/kg places it in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably smoother. The certificate is a use-point analysis: the sample was drawn at the large bath rather than at the wellhead, and the lab recorded a faint saltiness on the tongue five hours after sampling.
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⁺652mgP81 national85%
- Potassiumminor; matters for drinking limitsK⁺98.0mgP92 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺29.1mgP45 national4%
- LithiumLi⁺4.5mgP94 national2%
- MagnesiumMg²⁺3.9mgP44 national1.0%
- ManganeseMn²⁺0.50mgP66 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,047mgP82 national86%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻205mgP66 national12%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻36.6mgP22 national2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃154mgP70 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂45.6mgP87 national
- Metaarsenious acidHAsO₂1.8mgP94 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂161mgP85 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.