A sodium-and-iron chloride spring with bicarbonate softness layered in. It is genuinely iron-rich (about 80 mg/kg of Fe(II)), so the water turns faintly yellow and slightly turbid with a metallic taste once it meets the air. As a cold-mineral source in the low-20 °C range, it is reheated 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, 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⁺170mgP42 national56%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺80.1mgP100 national22%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺45.0mgP56 national17%
- Magnesiummild; a laxative if you drink itMg²⁺5.0mgP50 national3%
- PotassiumK⁺5.0mgP35 national1.0%
- AmmoniumNH₄⁺2.5mgP83 national0.5%
- Manganesetrace; companion to iron springsMn²⁺0.40mgP60 national
- LithiumLi⁺0.14mgP30 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻259mgP60 national53%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻394mgP71 national47%
- SulfateSO₄²⁻4.0mgP14 national0.2%
- Iodideyellows in air; helps blood lipidsI⁻0.30mgP62 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃22.6mgP7 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.8mgP72 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂467mgP95 national
- CopperCu0.02mgP86 national
- ArsenicAs0.0050mgP16 national
- CadmiumCd< —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.