An alkaline simple thermal spring (Yunoyama Kataoka Onsen in the foothills of Yunoyama) with a softly slippery, beauty-water character — pH 8.5 at the wellhead is enough to give the skin that characteristic slip. Lightly mineralized at 0.54 g/kg dissolved solids, the water is gentle on the body, with a faint carbonated tang on the tongue that the certificate notes both at the wellhead and three days later in the lab.
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⁺154mgP40 national96%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.1mgP15 national3%
- PotassiumK⁺1.6mgP19 national0.6%
- AluminumAl³⁺0.20mgP64 national0.3%
- Iron (II)Fe²⁺0.20mgP39 national0.1%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻244mgP61 national58%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻87.7mgP38 national36%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻6.0mgP39 national3%
- Fluoridetrace; drinking limits applyF⁻3.2mgP67 national2%
- BromideBr⁻0.20mgP30 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.8mgP18 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.3mgP39 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.1mgP18 national
- ArsenicAs0.0050mgP15 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.