A sodium-led bicarbonate spring with a chloride tail — classic jūsō-sen territory and exactly the chemistry that earned Ureshino its reputation as one of Japan's three great beauty-water springs. The water emerges remarkably hot (88.3 °C, among Japan's hottest natural sources) and is weakly alkaline, so the soak carries that signature slippery, skin-softening feel. Moderate mineralization (1,905 mg/kg dissolved) keeps the body light, while 140.6 mg/kg of metasilicic acid adds a faint moisturizing film.
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⁺462mgP74 national
- Potassiumminor; matters for drinking limitsK⁺27.0mgP72 national
- LithiumLi⁺1.3mgP85 national
- Magnesiummild; a laxative if you drink itMg²⁺0.70mgP23 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺0.0000mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻955mgP90 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻250mgP59 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻9.3mgP20 national
- Fluoridetrace; drinking limits applyF⁻9.0mgP89 national
- Bromidetrace, sea-originBr⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃141mgP67 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂42.0mgP85 national
- ArsenicAs0.0050mgP15 national
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.