A sodium-led chloride spring with a touch of bicarbonate softness layered in. On the posted composition table sodium dominates the cations while chloride is the leading anion, giving a salty, heat-retaining character. Only the ion-composition portion of the analysis is posted here, with no pH, source temperature, or formal spring-type name shown, so the finer sensory detail cannot be confirmed from this document.
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⁺1,042mgP86 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺32.5mgP48 national3%
- Potassiumminor; matters for drinking limitsK⁺1.0mgP14 national
- Magnesiummild; a laxative if you drink itMg²⁺0.09mgP7 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻100mgP40 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻38.7mgP23 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻25.4mgP77 national
- Fluoridetrace; drinking limits applyF⁻17.0mgP98 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.0mgP22 national
- BO2BO₂⁻5.4mgP83 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.30mgP47 national
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃66.7mgP42 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< 4.0mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
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.