A sodium-led chloride spring with bicarbonate softness layered in. Weakly alkaline and soft on the skin, it sits in the Japanese beauty-water family. The water carries a faint yellow-brown tint, a slight salty tang, and a faint hydrogen sulfide note, and the source also bears a small amount of radon.
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⁺380mgP68 national88%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺27.4mgP44 national7%
- Potassiumminor; matters for drinking limitsK⁺9.5mgP44 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.20mgP39 national
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP14 national
- StrontiumSr²⁺0.20mgP44 national
- LithiumLi⁺0.10mgP29 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- AmmoniumNH₄⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻467mgP73 national69%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻385mgP70 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.50mgP5 national
- Fluoridetrace; drinking limits applyF⁻0.30mgP24 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.10mgP28 national
- Iodideyellows in air; helps blood lipidsI⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃93.1mgP53 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.80mgP11 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂521mgP96 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.