A blended source drawing from Nakanoshima wells No. 4, 5, 6, and 7, emerging at a hot 52.5 C. The chemistry is sodium chloride water at 8,612 mg/kg dissolved solids — isotonic, weakly alkaline, and decisively salty on the lips. A faint rotten-egg note of hydrogen sulfide drifts off the source, and the heat-retaining mineral coating clings to the skin well after the bath.
Good for
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
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
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⁺2,540mgP92 national73%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺417mgP90 national14%
- Magnesiummild; a laxative if you drink itMg²⁺210mgP96 national12%
- Potassiumminor; matters for drinking limitsK⁺57.7mgP85 national1%
- 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
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,798mgP92 national91%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻580mgP91 national8%
- BicarbonateHCO₃⁻36.4mgP21 national0.4%
- Hydrogen sulfideHS⁻14.5mgP97 national0.3%
- FluorideF⁻4.4mgP77 national0.1%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃43.4mgP25 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.4mgP14 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂9.2mgP37 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.4mgP81 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.