地涌の泉 不老の山Jiyu-no-Izumi Furo-no-Yama Spring
A sodium chloride strong-salt spring drawn in the Koyama district of Shinagawa. It is hypertonic, carrying 18.6 g/kg of dissolved solids, and the certificate describes the water at the wellhead as faintly yellow and clear, briny on the tongue, with a trace of mineral-oil odor and gas bubbling up alongside the flow. Weakly alkaline, it also holds 105.7 mg/kg of metasilicic acid and an unusually high 24.3 mg/kg of iodide, so a soak leaves both the thick mineral coating prized in strong-salt water and the smoothing touch of Japan's beauty-water family.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺6,500mgP98 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺168mgP84 national3%
- Magnesiummild; a laxative if you drink itMg²⁺82.4mgP91 national2%
- Potassiumminor; matters for drinking limitsK⁺201mgP97 national2%
- AmmoniumNH₄⁺92.7mgP100 national2%
- Iron (II)Fe²⁺1.6mgP69 national0.0%
- AluminumAl³⁺0.20mgP65 national0.0%
- ManganeseMn²⁺0.20mgP46 national0.0%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10,510mgP97 national95%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻854mgP87 national5%
- BromideBr⁻56.5mgP99 national0.2%
- IodideI⁻24.3mgP99 national0.1%
- CarbonateCO₃²⁻2.1mgP22 national0.0%
- SulfateSO₄²⁻0.60mgP5 national0.0%
- FluorideF⁻0.10mgP8 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃106mgP56 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂10.0mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂49.7mgP68 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.