A sodium-chloride spring carrying a faint brown tint at the source and a mild salty tang on the lips. Weakly alkaline and soft on the skin, with 87 mg/kg of metasilicic acid contributing the kind of silica film associated with Japan's "beauty-water" family. Iodide is unusually high at 12.2 mg/kg — enough to qualify as an iodine-bearing spring, so the water deepens in colour on contact with air.
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⁺3,282mgP94 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺136mgP78 national4%
- Magnesiummild; a laxative if you drink itMg²⁺44.6mgP84 national2%
- Potassiumminor; matters for drinking limitsK⁺102mgP92 national2%
- Iron (II)Fe²⁺1.2mgP65 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,980mgP93 national88%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,114mgP91 national11%
- IodideI⁻12.2mgP98 national0.1%
- BromideBr⁻7.3mgP86 national0.1%
- Hydrogen phosphateHPO₄²⁻1.4mgP86 national0.0%
- SulfateSO₄²⁻0.20mgP3 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃87.0mgP50 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂40.7mgP85 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.3mgP30 national
- 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.
- 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.