A sodium-calcium chloride spring of low tonicity and weakly alkaline pH (38.2 °C at the wellhead). With 1,245 mg/kg of dissolved solids it is moderately mineralized, soft on the skin, with a very faint salty note. A generous 118 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺328mgP64 national77%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺75.0mgP65 national20%
- Potassiumminor; matters for drinking limitsK⁺16.4mgP56 national2%
- MagnesiumMg²⁺1.4mgP29 national0.7%
- LithiumLi⁺0.20mgP40 national0.2%
- StrontiumSr²⁺0.30mgP55 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- BariumBa²⁺0.0000mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.0000mg
- Iron (III)Fe³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- AmmoniumNH₄⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻553mgP76 national85%
- Hydrogen phosphateHPO₄²⁻79.2mgP98 national9%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻56.2mgP30 national5%
- HSiO3HSiO₃⁻5.4mgP56 national1.0%
- FluorideF⁻1.0mgP47 national0.3%
- Bromidetrace, sea-originBr⁻0.60mgP49 national
- AsO2AsO₂⁻0.0000mg
- BO2BO₂⁻0.0000mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- HSO4HSO40.0000mg
- Iodideyellows in air; helps blood lipidsI⁻0.0000mg
- HydroxideOH⁻0.0000mg
- SS²⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃118mgP60 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂10.1mgP56 national
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- CopperCu0.01mgP78 national
- ArsenicAs0.0070mgP18 national
- CadmiumCd< —mg
- ChromiumCr< —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.