A sodium-led bicarbonate spring — the classic 'jūsō-sen' (soda-water) style, neutral at pH 7.12. With 3,400 mg/kg of dissolved solids and a generous 90 mg/kg of metasilicic acid, it sits in the Japanese 'beauty-water' family — soft on the skin, with just the faintest salty edge on the tongue and otherwise no colour or scent. The unusual twist is a radon load of about 58 Bq/kg, putting it in the 'hormesis' tradition of radon springs as well.
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⁺799mgP82 national93%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺21.4mgP40 national3%
- Potassiumminor; matters for drinking limitsK⁺31.1mgP75 national2%
- Magnesiummild; a laxative if you drink itMg²⁺8.3mgP59 national2%
- Iron (II)Fe²⁺0.50mgP52 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.02mgP18 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,073mgP96 national80%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻295mgP63 national20%
- SulfateSO₄²⁻17.3mgP29 national0.8%
- BromideBr⁻1.1mgP62 national0.0%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- SS²⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃90.4mgP52 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂70.2mgP93 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂231mgP90 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- ArsenicAs< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.