Yumatsu no Yu, rising in Nakamachi, Izumisano, is a simple thermal spring classified hypotonic, neutral, and low-temperature. Its neutral pH makes for a gentle, low-irritation water, while a respectable 65.8 mg/kg of metasilicic acid leaves the skin feeling smooth. At 27.3 °C the source is a low-temperature spring, reheated for bathing.
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⁺37.2mgP8 national63%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.4mgP29 national24%
- Magnesiummild; a laxative if you drink itMg²⁺3.2mgP40 national10%
- Potassiumminor; matters for drinking limitsK⁺1.4mgP17 national1%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.80mgP59 national1%
- ManganeseMn²⁺0.10mgP36 national0.3%
- AmmoniumNH₄⁺0.10mgP24 national0.2%
- StrontiumSr²⁺0.10mgP26 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- LithiumLi⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻147mgP51 national85%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻13.8mgP15 national14%
- SulfateSO₄²⁻1.0mgP7 national0.7%
- Bromidetrace, sea-originBr⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Fluoridetrace; drinking limits applyF⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃65.8mgP41 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP19 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂20.0mgP52 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.