A sodium-led chloride spring with the gentle, mineral softness of a bicarbonate sub-type layered in. Neutral in pH and moderately mineralized at 1,279 mg/kg, the water at the source carries a faint rotten-egg note of hydrogen sulfide and a slightly bitter edge if tasted. With 69.5 mg/kg of metasilicic acid it has just enough silica to feel a touch smoother on the skin.
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⁺318mgP65 national79%
- StrontiumSr²⁺48.4mgP99 national14%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.4mgP27 national5%
- PotassiumK⁺6.3mgP37 national0.9%
- Iron (II)Fe²⁺3.1mgP79 national0.6%
- LithiumLi⁺0.30mgP41 national0.2%
- ManganeseMn²⁺0.40mgP61 national0.0%
- MagnesiumMg²⁺0.10mgP11 national0.0%
- AmmoniumNH₄⁺0.10mgP23 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻385mgP72 national60%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻421mgP73 national38%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻9.3mgP20 national1%
- Fluoridetrace; drinking limits applyF⁻3.6mgP68 national1%
- BromideBr⁻0.30mgP37 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃69.5mgP43 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.8mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂44.0mgP67 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- 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.
- 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.