A sodium-led bicarbonate spring from Tanoyu-machi in Beppu, classified as juso-sen (soda-water style, soft on the skin) with a salty edge from its chloride sub-type. Mildly alkaline and moderately mineralized, it has noticeable body while staying gentle, and sits in Japan's beauty-water family. Metasilicic acid at 192 mg/kg leaves a thin silica film that makes skin feel noticeably smoother afterwards.
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
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⁺255mgP59 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺31.2mgP46 national11%
- Magnesiummild; a laxative if you drink itMg²⁺13.5mgP66 national8%
- Potassiumminor; matters for drinking limitsK⁺21.4mgP65 national4%
- LithiumLi⁺1.9mgP90 national2%
- Iron (II)Fe²⁺0.70mgP59 national0.2%
- AmmoniumNH₄⁺0.30mgP39 national0.1%
- StrontiumSr²⁺0.30mgP52 national0.1%
- BariumBa²⁺0.20mgP72 national0.0%
- ManganeseMn²⁺0.10mgP37 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻488mgP76 national56%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻168mgP54 national33%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻67.8mgP47 national10%
- BromideBr⁻0.50mgP48 national0.1%
- FluorideF⁻0.10mgP8 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃192mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.2mgP52 national
- Metaarsenious acidHAsO₂0.40mgP78 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.9mgP34 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.29mgP86 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.