A cold-mineral spring (14.3 °C) at Ihatasan Yakushiji, reheated for bathing. Lightly mineralized at about 697 mg/kg, it is alkaline and soft on the skin, with notable fluoride and metasilicic acid carried alongside the bicarbonate framework. The certificate also approves it for drinking — one of the rarer sources that doubles as a fountain — and lists digestive complaints among its indications.
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
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, severe anemia or other marked physical weakness
- Severe heart or lung disease causing shortness of breath or palpitations on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Visible bleeding
- Worsening phase of a chronic illness
- Diarrhea
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⁺212mgP55 national98%
- CalciumCa²⁺1.2mgP4 national0.6%
- PotassiumK⁺1.4mgP16 national0.4%
- LithiumLi⁺0.20mgP37 national0.3%
- MagnesiumMg²⁺0.20mgP14 national0.2%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻367mgP70 national68%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻75.0mgP97 national28%
- Fluoridetrace; drinking limits applyF⁻4.5mgP75 national3%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8.3mgP11 national3%
- SulfateSO₄²⁻1.9mgP10 national0.4%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃20.0mgP5 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.0mgP52 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂13.2mgP45 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.