A sodium-magnesium chloride spring labeled 'kyo-ensen' (strong-salt) on the certificate — at 24.45 g/kg total dissolved it approaches ocean salinity. Clear at the wellhead but with a forceful salty-bitter taste and a faint iron note; the lab sample yellows and turns slightly cloudy within hours. Neutral pH, hypertonic, with a generous 168.5 mg/kg of metasilicic acid — bathers leave with a thick mineral coating clinging to the skin.
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⁺5,806mgP98 national62%
- Magnesiummild; a laxative if you drink itMg²⁺1,086mgP100 national22%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,074mgP95 national13%
- Potassiumminor; matters for drinking limitsK⁺310mgP98 national2%
- Iron (II)Fe²⁺16.5mgP93 national0.1%
- LithiumLi⁺4.3mgP94 national0.1%
- StrontiumSr²⁺13.7mgP94 national0.1%
- AmmoniumNH₄⁺3.1mgP86 national0.0%
- ManganeseMn²⁺1.2mgP80 national0.0%
- BariumBa²⁺0.40mgP81 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻12,760mgP97 national86%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,466mgP99 national7%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,657mgP94 national7%
- BromideBr⁻31.2mgP95 national0.1%
- IodideI⁻0.40mgP66 national0.0%
- H2PO4H₂PO₄⁻0.20mgP57 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃169mgP76 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂56.2mgP90 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂489mgP96 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.01mgP52 national
- CopperCu< —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.
- 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.