A sodium-magnesium-calcium bicarbonate-chloride spring, filed on the certificate under its older classification as a soda-water (jūsō-sen) type with an earthy-chloride character. Weakly alkaline and soft on the skin, it sits in Japan's beauty-water family, with a salty edge from the chloride side and moderate mineralization at 1,015 mg/kg. The standout figure is metasilicic acid at 259 mg/kg — enough silica to leave the thin, moisturizing film that makes skin feel noticeably smoother afterwards. The water itself is colorless, clear, tasteless 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
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⁺102mgP30 national42%
- Magnesiummild; a laxative if you drink itMg²⁺33.9mgP78 national26%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺54.4mgP59 national26%
- Potassiumminor; matters for drinking limitsK⁺15.3mgP53 national4%
- LithiumLi⁺1.1mgP76 national2%
- Iron (II)Fe²⁺0.50mgP52 national0.2%
- ManganeseMn²⁺0.50mgP64 national0.2%
- StrontiumSr²⁺0.30mgP55 national0.1%
- BariumBa²⁺0.20mgP74 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻383mgP70 national61%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻86.1mgP37 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻74.9mgP50 national15%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.1%
- BromideBr⁻0.20mgP30 national0.0%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃259mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.5mgP27 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂25.5mgP55 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.05mgP51 national
- 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.
- 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.