A municipal Beppu blend (Unsenji source plus others, sampled at a covered mixing-tank outlet) emerges hot at 64.1 °C as a simple thermal water, neutral and hypotonic. Total dissolved solids sit modestly at 0.764 g/kg with a faint sodium-chloride lean, so the water reads gentle rather than briny. The headline is silica — metasilicic acid at 159 mg/kg leaves the kind of fine film that makes skin feel noticeably smoother on the way out.
Good for
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
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⁺173mgP45 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.2mgP34 national9%
- Potassiumminor; matters for drinking limitsK⁺17.5mgP59 national5%
- Magnesiummild; a laxative if you drink itMg²⁺4.6mgP49 national4%
- LithiumLi⁺1.3mgP85 national2%
- StrontiumSr²⁺—mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻168mgP52 national55%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻142mgP50 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻71.2mgP48 national17%
- FluorideF⁻0.60mgP38 national0.3%
- NitrateNO₃⁻1.4mgP85 national0.2%
- BromideBr⁻0.50mgP47 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃159mgP73 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.8mgP52 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.90mgP17 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.