A hot 68 °C sodium bicarbonate-chloride spring, neutral pH and moderately mineralized at 1,583 mg/kg dissolved solids. A faint hydrogen sulfide note and slight bitterness on the lips give the water its character. With 135 mg/kg of metasilicic acid it leaves a thin silica film on the skin, while the bicarbonate softness and the salty edge from chloride layer cleanly across the soak.
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⁺385mgP69 national86%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺42.9mgP55 national11%
- Magnesiummild; a laxative if you drink itMg²⁺8.1mgP59 national3%
- Potassiumminor; matters for drinking limitsK⁺17.1mgP56 national2%
- Manganesetrace; companion to iron springsMn²⁺0.01mgP17 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻780mgP85 national68%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻195mgP53 national29%
- Fluoridetrace; drinking limits applyF⁻6.9mgP84 national2%
- Thiosulfateoxidized sulfurS₂O₃²⁻6.5mgP94 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻6.5mgP17 national
- BO2BO₂⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃135mgP66 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.6mgP60 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂54.9mgP70 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- ArsenicAs< —mg
- CopperCu< —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.