A gentle simple thermal spring: neutral in reaction and lightly mineralized at 947 mg/kg of dissolved solids, so it sits softly on the skin without any sharp edge. What lifts it above the ordinary is metasilicic acid at 230.4 mg/kg — a generous dose that leaves the thin silica film associated with Japan's beauty-water family, and skin feels noticeably smoother afterwards. Bicarbonate accounts for nearly three-quarters of the anions, giving the water a soda-like softness despite its plain classification.
Good for
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
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
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⁺110mgP31 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.9mgP54 national23%
- Magnesiummild; a laxative if you drink itMg²⁺21.9mgP71 national20%
- Potassiumminor; matters for drinking limitsK⁺12.6mgP49 national4%
- Iron (III)Fe³⁺0.50mgP78 national0.3%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻422mgP73 national73%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻47.2mgP27 national14%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻59.6mgP44 national13%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃230mgP91 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.2mgP20 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂40.6mgP65 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.
- 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.