A sodium-magnesium chloride cold-mineral spring, with bicarbonate softness layered in. Weakly alkaline and soft on the skin, it carries a faint brown tint with rising bubbles at the source. With 64 mg/kg of metasilicic acid it leaves a thin silica film that sits in the Japanese beauty-water family. As a cold-mineral source under 25 °C, it is heated for bathing.
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, netto 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⁺233mgP58 national62%
- Magnesiummild; a laxative if you drink itMg²⁺40.0mgP83 national20%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺36.2mgP49 national11%
- Potassiumminor; matters for drinking limitsK⁺29.3mgP74 national5%
- AmmoniumNH₄⁺4.6mgP90 national2%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻339mgP69 national60%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻343mgP68 national35%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1.0mgP7 national3%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻6.6mgP38 national1%
- Bromidetrace, sea-originBr⁻0.20mgP28 national
- Fluoridetrace; drinking limits applyF⁻0.10mgP8 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃64.0mgP39 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.2mgP30 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- 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.