A sodium bicarbonate cold-mineral spring of the classic Japanese beauty-water family. Alkaline at pH 8.9, it has the slippery, skin-smoothing feel typical of bicarbonate water. The source emerges at just 21.6 °C, below the 25 °C threshold, so it is reheated for bathing. Fluoride is also listed among the components that qualify it as an onsen under the Hot Spring Law.
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
Take care
- General contraindications for bathing: acute phase of illness (especially with fever), active tuberculosis, advanced malignant tumors or marked physical debilitation such as severe anemia, serious heart or lung disease causing breathlessness even on slight exertion, severe kidney disease with edema, gastrointestinal bleeding, visible hemorrhage, and acute exacerbation of a chronic illness.
- No applicable contraindications listed for drinking.
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⁺149mgP40 national98%
- CalciumCa²⁺1.2mgP4 national0.9%
- PotassiumK⁺1.4mgP17 national0.6%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< 0.10mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< 0.10mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< 0.10mg
- Manganesetrace; companion to iron springsMn²⁺< 0.10mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻288mgP64 national72%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻40.5mgP88 national21%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.0mgP22 national4%
- Fluoridetrace; drinking limits applyF⁻2.6mgP64 national2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3.1mgP4 national1%
- HydroxideOH⁻0.20mgP66 national0.1%
- BO2BO₂⁻< 0.10mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< 0.10mg
- HSiO3HSiO₃⁻< 0.10mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< 0.10mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃13.8mgP2 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.2mgP20 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.50mgP14 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< 0.10mg
- CadmiumCd0.05mgP95 national
- LeadPb0.05mgP85 national
- CopperCu0.01mgP76 national
- ArsenicAs0.0010mgP6 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0005mgP51 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.