A sodium chloride spring in strong-salt territory — dissolved solids of 12,620 mg/kg sit well above ocean salinity and leave a thick mineral coating on the skin. At a wellhead temperature of 75 °C it ranks among Japan's hottest natural sources, with a faint hydrogen sulfide note and a moderate salty tang at the source. Metasilicic acid at 95 mg/kg adds a thin silica film under the salt, softening the finish.
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⁺4,004mgP96 national85%
- Magnesiummild; a laxative if you drink itMg²⁺238mgP97 national10%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺118mgP75 national3%
- Potassiumminor; matters for drinking limitsK⁺192mgP97 national2%
- AluminumAl³⁺0.40mgP76 national0.0%
- ManganeseMn²⁺0.70mgP77 national0.0%
- Iron (II)Fe²⁺0.10mgP32 national0.0%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,285mgP92 national76%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,050mgP95 national17%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻583mgP90 national6%
- FluorideF⁻3.7mgP71 national0.1%
- CarbonateCO₃²⁻3.9mgP29 national0.1%
- Hydrogen sulfideHS⁻0.40mgP50 national0.0%
- ThiosulfateS₂O₃²⁻0.20mgP52 national0.0%
- BO2BO₂⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃95.3mgP56 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂49.4mgP87 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂70.4mgP73 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP62 national
- ArsenicAs< —mg
- 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.