A sodium-led bicarbonate spring in the classic jūsō-sen style, with a salt-tinged finish from a chloride sub-type. Weakly alkaline and soft on the skin, it sits squarely in Japan's beauty-water family. A generous 120 mg/kg of metasilicic acid leaves a thin silica film that makes the skin feel noticeably smoother afterwards, while a barely-there hydrogen sulfide note hangs over the bath.
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
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺350mgP66 national89%
- Magnesiummild; a laxative if you drink itMg²⁺9.5mgP60 national5%
- Potassiumminor; matters for drinking limitsK⁺23.0mgP67 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.0mgP27 national3%
- LithiumLi⁺0.20mgP40 national0.2%
- Iron (II)Fe²⁺0.20mgP39 national0.1%
- BariumBa²⁺0.20mgP74 national
- StrontiumSr²⁺0.07mgP15 national
- Manganesetrace; companion to iron springsMn²⁺0.06mgP23 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺NDmg
- Iron (III)Fe³⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻780mgP85 national75%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻150mgP48 national25%
- CarbonateCO₃²⁻2.8mgP27 national0.5%
- BromideBr⁻0.50mgP47 national0.1%
- IodideI⁻0.0000mg0.0%
- Fluoridetrace; drinking limits applyF⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃120mgP61 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂27.0mgP78 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂32.0mgP62 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.07mgP59 national
- 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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.