A sodium-calcium chloride spring — salty and heat-retaining, the kind of water that holds warmth well after the bath. Weakly alkaline and soft on the skin, it carries a fair amount of metasilicic acid (62 mg/kg) that lends a smoother feel. At the source the water shows a faint yellow tint and is slightly turbid.
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⁺446mgP73 national72%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺115mgP75 national21%
- Magnesiummild; a laxative if you drink itMg²⁺14.0mgP66 national4%
- Potassiumminor; matters for drinking limitsK⁺12.8mgP50 national1%
- Iron (II)Fe²⁺1.7mgP69 national0.2%
- BariumBa²⁺1.7mgP93 national0.1%
- StrontiumSr²⁺1.0mgP78 national0.1%
- Iron (III)Fe³⁺2.4mgP91 national0.0%
- ManganeseMn²⁺0.40mgP59 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.06mgP31 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻797mgP79 national85%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻232mgP59 national14%
- BromideBr⁻2.0mgP71 national0.1%
- IodideI⁻3.1mgP90 national0.1%
- HSO4HSO40.80mgP44 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃62.0mgP38 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.60mgP10 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂17.6mgP50 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.01mgP25 national
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.