A sodium-calcium chloride spring drawn at the Hashima municipal senior welfare centre. Neutral in pH and moderately mineralized at 5,081 mg/kg of dissolved solids, it has the salty, heat-retaining body typical of chloride water — the kind of warmth that lingers well after you climb out. The posted sensory test records the water as colorless and clear, with a faint metallic iron note and a slight salty tang, the signature of the small amount of iron(II) it carries.
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
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⁺907mgP84 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺740mgP95 national
- Magnesiummild; a laxative if you drink itMg²⁺124mgP93 national
- Potassiumminor; matters for drinking limitsK⁺16.6mgP56 national
- Manganesetrace; companion to iron springsMn²⁺1.9mgP90 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.5mgP69 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.04mgP28 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,092mgP91 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻149mgP52 national
- Fluoridetrace; drinking limits applyF⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃27.0mgP10 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.1mgP12 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.4mgP50 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.