A sodium chloride-sulfate spring rising on the Kannawa hillside in Beppu. Weakly acidic and gentle on the skin, it pairs the heat-retaining pull of chloride with the quiet body that sulfate adds, and its moderate mineralization gives the water noticeable substance. A generous 177 mg/kg of metasilicic acid puts it in Japan's beauty-water family — the kind of silica film that leaves skin feeling smoother afterwards. Colorless and clear at the wellhead, the sample had taken on a very faint yellow by the time the lab re-examined it seven hours later. The certificate lists cuts, burns and chronic skin conditions among the spring-type indications, and the inn's own panel shows this water feeding six private rooftop family baths.
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
Take care
- Acute illness (especially with fever)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Respiratory failure
- Renal failure
- Bleeding disorders
- Severe anemia
- Other conditions in an actively progressing stage
- Pregnancy (especially early and late stages)
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⁺290mgP62 national79%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺26.2mgP43 national8%
- Potassiumminor; matters for drinking limitsK⁺40.5mgP82 national7%
- Magnesiummild; a laxative if you drink itMg²⁺6.9mgP57 national4%
- LithiumLi⁺2.3mgP92 national2%
- ManganeseMn²⁺0.90mgP75 national0.2%
- Iron (II)Fe²⁺0.70mgP59 national0.2%
- AluminumAl³⁺0.10mgP55 national0.1%
- StrontiumSr²⁺0.10mgP24 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻417mgP73 national74%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻189mgP64 national25%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻12.2mgP8 national1%
- FluorideF⁻0.60mgP35 national0.2%
- BromideBr⁻1.2mgP64 national0.1%
- NitrateNO₃⁻1.2mgP79 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃177mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂18.7mgP73 national
- Metaarsenious acidHAsO₂0.30mgP74 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂11.0mgP40 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.23mgP82 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic digestive disorders
- Chronic skin conditions
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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.