A simple thermal spring delivered on Beppu City's Fujimi supply line, sampled not at a wellhead but at the outlet of a storage tank that pools water from more than one source. It is neutral and lightly mineralized at 723 mg/kg, colorless, clear, tasteless and odorless, so it feels gentle rather than assertive on the skin. What lifts it out of the ordinary is metasilicic acid at 151 mg/kg, generous enough to put it in Japan's beauty-water family and to leave the thin silica film that makes skin feel noticeably smoother afterwards.
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⁺157mgP42 national79%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺15.4mgP33 national9%
- Magnesiummild; a laxative if you drink itMg²⁺5.7mgP54 national5%
- Potassiumminor; matters for drinking limitsK⁺16.3mgP55 national5%
- LithiumLi⁺1.2mgP79 national2%
- StrontiumSr²⁺0.20mgP44 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻154mgP49 national53%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻138mgP48 national28%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻74.4mgP49 national19%
- FluorideF⁻0.50mgP32 national0.4%
- NitrateNO₃⁻1.5mgP87 national0.2%
- BromideBr⁻0.50mgP47 national0.1%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃151mgP68 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.2mgP47 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.0mgP27 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.12mgP67 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.