A sodium-led bicarbonate spring with a chloride sub-type — the classic jūsō-sen (soda-water) style, soft and smooth on the skin. With neutral pH and a generous 124.6 mg/kg of metasilicic acid, it sits squarely in Japan's beauty-water family, leaving the kind of silica film that makes skin feel noticeably smoother. Moderately mineralized at 1,274.6 mg/kg of dissolved solids, the water is colorless, transparent and odorless.
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⁺300mgP62 national89%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺23.9mgP42 national8%
- Magnesiummild; a laxative if you drink itMg²⁺3.6mgP42 national2%
- Potassiumminor; matters for drinking limitsK⁺7.7mgP40 national1%
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- LithiumLi⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻649mgP82 national70%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻134mgP45 national25%
- Fluoridetrace; drinking limits applyF⁻7.8mgP86 national3%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.1mgP26 national2%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃125mgP64 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.3mgP53 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂76.0mgP76 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.