A sodium chloride cold-mineral spring that is also markedly iodine-rich. The salty, heat-retaining chloride water is moderately to richly mineralized at 9,050 mg/kg of dissolved solids, leaving a mineral film on the skin. Because the source emerges below 25 °C, it is a cold-mineral spring heated for bathing. With iodide at 22.2 mg/kg the water belongs to the iodine-bearing family and tends to discolour 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⁺3,172mgP93 national94%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺127mgP78 national4%
- Magnesiummild; a laxative if you drink itMg²⁺29.3mgP75 national2%
- PotassiumK⁺12.1mgP48 national0.2%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.9mgP71 national
- Manganesetrace; companion to iron springsMn²⁺0.29mgP45 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.20mgP64 national
- CopperCu²⁺0.0020mgP43 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,449mgP92 national86%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻988mgP90 national11%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻177mgP63 national3%
- BromideBr⁻48.4mgP99 national0.4%
- IodideI⁻22.2mgP98 national0.1%
- CarbonateCO₃²⁻1.2mgP21 national0.0%
- FluorideF⁻0.30mgP24 national0.0%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂16.5mgP70 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃5.7mgP1 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂114mgP81 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAsNDmg
- CadmiumCdNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.