A blended simple thermal spring: the certificate names Beppu City's Unsenji source and others, with the sample drawn at the outlet of the storage tank where they are pooled. Neutral and hypotonic, it carries only 764 mg/kg of dissolved solids, so the water itself is light and undemanding on the body. What lifts it is silica — 159 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the kind of thin mineral 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⁺173mgP45 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.2mgP34 national9%
- Potassiumminor; matters for drinking limitsK⁺17.5mgP58 national5%
- Magnesiummild; a laxative if you drink itMg²⁺4.6mgP49 national4%
- LithiumLi⁺1.3mgP85 national2%
- StrontiumSr²⁺0.20mgP42 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻168mgP52 national55%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻142mgP51 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻71.2mgP47 national17%
- FluorideF⁻0.60mgP36 national0.3%
- NitrateNO₃⁻1.4mgP87 national0.2%
- BromideBr⁻0.50mgP48 national0.1%
- Hydrogen phosphateHPO₄²⁻0.20mgP53 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃159mgP73 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.8mgP51 national
- Metaarsenious acidHAsO₂0.20mgP67 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.90mgP17 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.16mgP76 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.