A sodium-led bicarbonate spring with a chloride sub-type — the jūsō-sen style, with a salty edge layered in. Weakly alkaline and soft on the skin, moderately mineralized at 1,566 mg/kg, it sits in Japan's beauty-water family. At the wellhead the water is faintly yellow-brown and clear, slightly salty on the lips, with a faint hydrogen sulfide note that has already faded by the time the sample reaches the lab. Metasilicic acid at 164.8 mg/kg leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺334mgP64 national79%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺33.8mgP48 national9%
- Magnesiummild; a laxative if you drink itMg²⁺16.1mgP68 national7%
- Potassiumminor; matters for drinking limitsK⁺29.9mgP74 national4%
- AmmoniumNH₄⁺1.6mgP76 national0.5%
- StrontiumSr²⁺0.40mgP62 national0.1%
- Iron (III)Fe³⁺0.20mgP66 national0.1%
- ManganeseMn²⁺0.20mgP45 national0.1%
- BariumBa²⁺0.20mgP74 national0.0%
- Iron (II)Fe²⁺0.10mgP30 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻706mgP83 national60%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻265mgP60 national39%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻8.1mgP45 national1%
- Hydrogen phosphateHPO₄²⁻1.0mgP82 national0.1%
- BromideBr⁻0.90mgP57 national0.1%
- ThiosulfateS₂O₃²⁻0.20mgP55 national0.0%
- SulfateSO₄²⁻< 0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃165mgP75 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.7mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂79.5mgP76 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< 0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.