A sodium-led bicarbonate spring rising in Kitahama, Beppu, listed on its certificate under the old name of pure juso-sen: the soda-water style that feels soft on the skin. Mildly alkaline and moderately mineralized with noticeable body, it belongs to Japan's beauty-water family. Metasilicic acid at 251 mg/kg is generous and 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
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⁺245mgP58 national66%
- Magnesiummild; a laxative if you drink itMg²⁺34.8mgP80 national18%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺34.9mgP49 national11%
- Potassiumminor; matters for drinking limitsK⁺24.3mgP69 national4%
- LithiumLi⁺1.7mgP88 national1%
- Iron (II)Fe²⁺0.60mgP57 national0.1%
- AmmoniumNH₄⁺0.40mgP43 national0.1%
- StrontiumSr²⁺0.30mgP52 national0.1%
- BariumBa²⁺0.20mgP72 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻747mgP85 national73%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻113mgP44 national19%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻63.4mgP46 national8%
- FluorideF⁻0.20mgP16 national0.1%
- BromideBr⁻0.30mgP37 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃251mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.5mgP47 national
- Metaarsenious acidHAsO₂0.20mgP67 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂44.9mgP68 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.17mgP77 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.
- 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.