A sodium- and magnesium-led bicarbonate-chloride spring, filed on the certificate under an older name that puts it in the jusou-sen (soda-water) family. Weakly alkaline and soft on the skin, it sits squarely in Japan's beauty-water family, and at 1,903 mg/kg of dissolved solids it has real body, with a very faint salty note on the tongue from the chloride side. Metasilicic acid runs to 231 mg/kg, enough to leave the thin silica film that makes skin feel noticeably smoother afterwards. The posted list of bathing indications covers cuts, burns and chronic skin conditions.
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
Take care
- Acute illness (especially with fever)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Bleeding disorders
- Severe anemia
- Respiratory failure
- Renal failure
- Any other illness in an actively progressing stage
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⁺282mgP60 national55%
- Magnesiummild; a laxative if you drink itMg²⁺57.4mgP86 national21%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺73.8mgP65 national17%
- Potassiumminor; matters for drinking limitsK⁺49.6mgP84 national6%
- LithiumLi⁺1.3mgP85 national0.9%
- AmmoniumNH₄⁺0.60mgP53 national0.1%
- StrontiumSr²⁺0.50mgP68 national0.1%
- ManganeseMn²⁺0.40mgP59 national0.1%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- BariumBa²⁺0.20mgP74 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻912mgP88 national67%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻189mgP53 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻94.5mgP54 national9%
- FluorideF⁻0.30mgP24 national0.1%
- BromideBr⁻0.50mgP47 national0.0%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- ThiosulfateS₂O₃²⁻0.20mgP55 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃231mgP91 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.9mgP52 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂61.6mgP73 national
- ArsenicAs0.21mgP81 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic skin conditions
- Chronic gynecological conditions
- Chronic digestive disorders
- Cold sensitivity
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.