A gentle simple thermal spring: Kiyousen draws on the No.1 and No.2 wells in Beppu's Kamegawa district and feeds them together into the bath. Neutral in pH and lightly mineralized at 0.73 g/kg of dissolved solids, it makes for an unfussy, low-irritation soak. What lifts it is silica — metasilicic acid at 193 mg/kg, enough to leave the thin moisturizing film that puts this water in Japan's beauty-water family. Note that the sample for this analysis was taken at the bath inlet rather than at the wellhead, so the figures describe the water as it actually arrives in the tub.
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⁺141mgP38 national75%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.7mgP35 national10%
- Potassiumminor; matters for drinking limitsK⁺23.8mgP68 national7%
- Magnesiummild; a laxative if you drink itMg²⁺6.6mgP56 national7%
- LithiumLi⁺0.70mgP64 national1%
- StrontiumSr²⁺0.10mgP26 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻122mgP44 national46%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻102mgP55 national28%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻115mgP46 national25%
- NitrateNO₃⁻3.0mgP94 national0.7%
- FluorideF⁻0.30mgP24 national0.3%
- BromideBr⁻0.40mgP39 national0.1%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃193mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.3mgP39 national
- Metaarsenious acidHAsO₂0.50mgP83 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.36mgP89 national
- LeadPb0.01mgP46 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.