A sodium- and magnesium-led bicarbonate-chloride spring in Beppu — the jūsō-sen (soda-water) style, soft on the skin, with a salty edge layered in from the chloride sub-type. Mildly alkaline and moderately mineralized at 1,900 mg/kg, it comes out of the well very faintly yellow but clear, and both tasteless and odorless. What sets it apart is metasilicic acid at 201.2 mg/kg — enough silica to leave a thin moisturizing film, which places it squarely in Japan's beauty-water family.
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, nettō 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⁺294mgP61 national54%
- Magnesiummild; a laxative if you drink itMg²⁺60.1mgP87 national21%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺90.5mgP69 national19%
- Potassiumminor; matters for drinking limitsK⁺47.9mgP84 national5%
- LithiumLi⁺1.2mgP79 national0.7%
- AmmoniumNH₄⁺0.50mgP49 national0.1%
- StrontiumSr²⁺0.70mgP74 national0.1%
- Iron (II)Fe²⁺0.50mgP52 national0.1%
- ManganeseMn²⁺0.50mgP64 national0.1%
- BariumBa²⁺0.40mgP81 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻786mgP86 national54%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻343mgP68 national40%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻66.4mgP46 national6%
- BromideBr⁻1.1mgP62 national0.0%
- Hydrogen phosphateHPO₄²⁻0.70mgP77 national0.0%
- FluorideF⁻0.10mgP9 national0.0%
- NitrateNO₃⁻0.30mgP43 national0.0%
- IodideI⁻0.10mgP37 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃201mgP85 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.8mgP41 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂17.6mgP50 national
- 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.