A sodium-led bicarbonate spring with a salty chloride edge, of the classic juso-sen (soda-water) style. Weakly alkaline and soft on the skin, it sits in Japan's beauty-water family, and 139.7 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother. The onsen karte posted at the facility calls it the typical soda-and-earth-salt water of the Beppu Station area, and its graders noted pale yellow, clear water with a faint sweet, oily moor-like aroma and a distinct slippery feel. Water is added only while the bath fills, because the source runs hot, and after that only source water flows in; this analysis was sampled in the bath itself. It is documented for cuts, burns and chronic skin conditions, and is for bathing only, not drinking.
Good for
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
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
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
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
From a certificate analyzed in 2004.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were removed from the official list of contraindications then, or include a condition that was. If you have a health condition, ask a doctor before bathing.
- Acute illness (especially when fever is present)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Respiratory failure
- Renal failure
- Bleeding disorders
- Severe anemia
- Any illness in an actively progressing stage
- Pregnancy (especially the early and late stages) (dropped from the official list in 2014)
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⁺399mgP71 national84%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺33.1mgP48 national8%
- Potassiumminor; matters for drinking limitsK⁺41.4mgP82 national5%
- Magnesiummild; a laxative if you drink itMg²⁺8.2mgP59 national3%
- ManganeseMn²⁺0.10mgP37 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻702mgP84 national53%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻308mgP66 national40%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻30.0mgP83 national5%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻21.0mgP33 national2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃140mgP66 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.4mgP42 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂44.0mgP67 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.0080mgP20 national
Measured at the source
On the certificate
From a certificate analyzed in 2004.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were dropped from the official list then.
- Chronic skin conditions (dropped from the official list in 2014)
- Chronic gynecological conditions (dropped from the official list in 2014)
- Chronic digestive disorders
- Hemorrhoids
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.