A sodium-led chloride spring — weakly alkaline (pH 8.18), with the salty, heat-retaining body characteristic of Na-Cl water. Total dissolved solids around 1,098 mg/kg place it in the moderately mineralized range, with chloride dominating the anion balance at nearly 90% by mval.
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⁺347mgP65 national91%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺23.3mgP41 national7%
- PotassiumK⁺6.1mgP37 national1.0%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.40mgP63 national
- LithiumLi⁺0.40mgP50 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP54 national
- Magnesiummild; a laxative if you drink itMg²⁺0.10mgP11 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- AmmoniumNH₄⁺0.10mgP24 national
- StrontiumSr²⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻540mgP75 national89%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻13.9mgP26 national2%
- Fluoridetrace; drinking limits applyF⁻4.3mgP74 national1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻98.7mgP98 national
- Bromidetrace, sea-originBr⁻0.40mgP39 national
- Iodideyellows in air; helps blood lipidsI⁻0.10mgP37 national
- SS²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃17.0mgP3 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.4mgP26 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
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.