An iodine-bearing spring defined by an unusually high iodide content — water that yellows on contact with air and carries a faint yellow-brown tint at the source. Weakly alkaline and soft on the skin, it shows a faint rotten-egg note of hydrogen sulfide and a slightly salty tang on the lips. Modest free CO2 and metasilicic acid round out a gentle, smooth feel. Iodine-rich waters are traditionally noted in connection with skin conditions, and the cert flags hyperthyroidism as a drinking caution for this water family.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺13.2mgP30 national41%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺7.7mgP2 national21%
- Magnesiummild; a laxative if you drink itMg²⁺3.6mgP42 national19%
- Aluminumshows up in acidic volcanic waterAl³⁺1.2mgP78 national8%
- Potassiumminor; matters for drinking limitsK⁺4.4mgP33 national7%
- AmmoniumNH₄⁺0.80mgP61 national3%
- FeTotalFe0.70mgP67 national2%
- Iodideyellows in air; helps blood lipidsI⁻3,846mgP100 national79%
- H2PO4H₂PO₄⁻350mgP100 national5%
- ChlorideCl⁻5.2mgP7 national0.2%
- SulfateSO₄²⁻12.3mgP24 national0.1%
- ThiosulfateS₂O₃²⁻3.8mgP92 national0.0%
- NitrateNO₃⁻0.30mgP43 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃107mgP57 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂108mgP81 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.0015mgP84 national
- ArsenicAs< —mg
- CopperCu< —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.