A sodium-led bicarbonate spring — the classic juso-sen (soda-water) style. Bicarbonate dominates at 2,093 mg/kg, far ahead of every other ion, and the mildly alkaline water is soft and faintly slippery on the skin: this sits squarely in Japan's beauty-water family, as the source name Bijin no Yu Shirotori ("beauty spring") announces. Dissolved solids reach 3,119 mg/kg — moderately mineralized, with noticeable body to the water.
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
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⁺874mgP84 national
- Potassiumminor; matters for drinking limitsK⁺9.6mgP45 national
- LithiumLi⁺2.5mgP93 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.1mgP3 national
- Magnesiummild; a laxative if you drink itMg²⁺0.90mgP25 national
- StrontiumSr²⁺0.90mgP79 national
- BariumBa²⁺0.80mgP85 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,093mgP97 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻78.3mgP33 national
- Fluoridetrace; drinking limits applyF⁻6.4mgP82 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻2.7mgP12 national
- Bromidetrace, sea-originBr⁻0.30mgP37 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃40.8mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.3mgP46 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂36.2mgP63 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.