A sodium-magnesium chloride-bicarbonate cold mineral spring drawn from beneath Asakusa in central Tokyo. At under 25 °C it is a cold-mineral source, heated for bathing, and the certificate records bubbles rising at the wellhead. The water is pale brown and clear — it carries 10 mg/kg of humic matter — and is almost tasteless and odorless. Mildly alkaline and soft on the skin, with bicarbonate softness layered over the salty base and a generous helping of metasilicic acid, it belongs to Japan's beauty-water family.
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⁺233mgP57 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⁺4.6mgP91 national2%
- AluminumAl³⁺0.30mgP66 national0.2%
- Iron (II)Fe²⁺0.20mgP39 national0.1%
- ManganeseMn²⁺0.20mgP45 national0.1%
- ZincZn²⁺0.04mgP67 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻339mgP68 national60%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻343mgP68 national35%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻21.9mgP31 national3%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻6.6mgP39 national1%
- Hydrogen phosphateHPO₄²⁻1.2mgP83 national0.2%
- BromideBr⁻1.1mgP61 national0.1%
- IodideI⁻0.20mgP51 national0.0%
- Fluoridetrace; drinking limits applyF⁻0.08mgP4 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃64.0mgP38 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.