A hot (64 C) simple thermal spring from the Unsenji area of Beppu — neutral and hypotonic, gentle on the body, presenting at the wellhead as colorless, transparent, tasteless and odorless. Mineralization is light at 764 mg/kg, but a generous 159 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother after the bath.
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⁺173mgP43 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.2mgP35 national9%
- Potassiumminor; matters for drinking limitsK⁺17.5mgP60 national5%
- Magnesiummild; a laxative if you drink itMg²⁺4.6mgP49 national4%
- LithiumLi⁺1.3mgP84 national2%
- StrontiumSr²⁺0.20mgP41 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻168mgP52 national55%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻142mgP49 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻71.2mgP50 national17%
- FluorideF⁻0.60mgP38 national0.3%
- Hydrogen phosphateHPO₄²⁻1.4mgP84 national0.2%
- BromideBr⁻0.50mgP47 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃159mgP75 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.8mgP49 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.90mgP16 national
- CopperCu0.60mgP100 national
- LeadPb0.16mgP97 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn< —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.