A sodium-led bicarbonate spring with a salt-tinged finish from a chloride sub-type, classified as a cold-mineral spring drawn at 22.7 °C and heated for bathing. Neutral in pH and soft on the skin, it carries metasilicic and metaboric acid alongside dissolved gas that bubbles up at the wellhead. With around 2,500 mg/kg of dissolved solids it is moderately mineralized, giving the water noticeable body.
Good for
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
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
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⁺553mgP77 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺57.8mgP60 national10%
- Magnesiummild; a laxative if you drink itMg²⁺31.0mgP76 national8%
- LithiumLi⁺2.4mgP93 national1%
- Potassiumminor; matters for drinking limitsK⁺12.1mgP48 national1%
- StrontiumSr²⁺1.0mgP78 national0.1%
- AmmoniumNH₄⁺0.10mgP24 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,141mgP92 national64%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻364mgP69 national35%
- FluorideF⁻2.7mgP64 national0.5%
- SulfateSO₄²⁻0.90mgP7 national0.1%
- Bromidetrace, sea-originBr⁻0.90mgP57 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.50mgP16 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃77.2mgP46 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂56.5mgP90 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂212mgP89 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.03mgP40 national
- 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.