A sodium-chloride strong-salt spring, hypertonic and densely mineralized. It surfaces at 65.8 °C — among Japan\u0027s hottest natural sources — clear at the wellhead but unmistakably salty on the lips. Dissolved solids reach 19.3 g/kg, exceeding many seawater readings, and bathers come away with a noticeable mineral film clinging to the skin.
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⁺5,936mgP98 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺223mgP86 national4%
- Potassiumminor; matters for drinking limitsK⁺236mgP98 national2%
- Magnesiummild; a laxative if you drink itMg²⁺53.3mgP85 national1%
- ZincZn²⁺0.01mgP41 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻9,412mgP96 national84%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,807mgP95 national9%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,017mgP95 national7%
- FluorideF⁻3.7mgP69 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃80.0mgP47 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂47.5mgP87 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂284mgP92 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —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.