A gentle simple thermal spring of Yunoura Onsen, emerging at 39.3 °C. Weakly alkaline and hypotonic, it sits well within the soft, easy-on-the-skin family of Japanese simple springs. Total dissolved solids are light at 494 mg/kg, and the wellhead carries a faintly sweet taste with a soft hydrocarbon note — an unobtrusive, settled water rather than an assertive one.
Good for
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⁺139mgP38 national86%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺13.7mgP30 national10%
- Potassiumminor; matters for drinking limitsK⁺4.2mgP32 national2%
- LithiumLi⁺0.70mgP64 national1%
- MagnesiumMg²⁺0.80mgP24 national0.9%
- StrontiumSr²⁺0.90mgP77 national0.3%
- ZincZn²⁺0.10mgP72 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻142mgP47 national54%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻178mgP55 national40%
- Fluoridetrace; drinking limits applyF⁻1.5mgP54 national1%
- Hydrogen sulfideHS⁻0.80mgP66 national0.5%
- SulfateSO₄²⁻0.60mgP5 national0.2%
- BromideBr⁻0.50mgP47 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻8.4mgP45 national
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃47.6mgP27 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.8mgP36 national
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.
- 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.