A sodium chloride strong-salt spring carrying iodine — a dense, hypertonic water. At 22,140 mg/kg of dissolved solids it exceeds ocean salinity and leaves a thick mineral coating on the skin. Neutral in pH, it tastes strongly salty with a metallic iron edge, and as an iodine-bearing source it turns faintly yellow and cloudy 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
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⁺7,450mgP99 national85%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺626mgP94 national8%
- Magnesiummild; a laxative if you drink itMg²⁺199mgP96 national4%
- Potassiumminor; matters for drinking limitsK⁺244mgP98 national2%
- AmmoniumNH₄⁺23.8mgP98 national0.3%
- Iron (II)Fe²⁺3.1mgP77 national0.0%
- StrontiumSr²⁺2.9mgP87 national0.0%
- ManganeseMn²⁺1.6mgP88 national0.0%
- LithiumLi⁺0.50mgP55 national0.0%
- BariumBa²⁺1.2mgP91 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻13,170mgP98 national99%
- BicarbonateHCO₃⁻226mgP59 national1.0%
- BromideBr⁻70.1mgP100 national0.2%
- IodideI⁻23.8mgP99 national0.1%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- FluorideF⁻0.10mgP9 national0.0%
- NitrateNO₃⁻0.04mgP20 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃91.9mgP52 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂10.0mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂12.6mgP43 national
- ArsenicAs< —mg
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.