A sodium-led chloride spring, weakly alkaline at the wellhead (pH 7.9) and pH 8.06 in the lab — soft on the skin and squarely in the Japanese beauty-water family. It carries 76.4 mg/kg of metasilicic acid, leaving the kind of thin silica film that makes skin feel smoother. At 31.2 °C it is a cold-mineral/low-temperature source, reheated for bathing.
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, nettō 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⁺314mgP63 national78%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺69.8mgP64 national20%
- PotassiumK⁺6.8mgP39 national1.0%
- LithiumLi⁺0.50mgP55 national0.4%
- StrontiumSr²⁺0.40mgP62 national0.1%
- ManganeseMn²⁺0.20mgP45 national0.1%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- AmmoniumNH₄⁺0.10mgP24 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻524mgP75 national83%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻102mgP43 national9%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻59.6mgP44 national7%
- FluorideF⁻2.5mgP63 national0.7%
- BromideBr⁻1.4mgP66 national0.1%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃76.4mgP46 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.3mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.0mgP51 national
- 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.