A sodium chloride strong-salt spring, exceeding ocean salinity and leaving a thick mineral veil on the skin after the bath. The wellhead runs at 100.4 °C — among Japan's hottest natural sources, cooled before bathing — and the water reads colorless and clear at the source, faintly turbid and off-white in the lab, with a pronounced salty tang and no odor. Neutral pH belies a heavily mineralized brine: total dissolved solids reach 34,240 mg/kg, and an unusually generous 587.6 mg/kg of metasilicic acid layers a thin silica film over the salt, lending the water a smoother finish than the headline chemistry would suggest.
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⁺10,270mgP99 national77%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,791mgP97 national15%
- Potassiumminor; matters for drinking limitsK⁺1,526mgP99 national7%
- LithiumLi⁺11.1mgP99 national0.3%
- StrontiumSr²⁺10.6mgP94 national0.0%
- AmmoniumNH₄⁺4.0mgP89 national0.0%
- ManganeseMn²⁺3.3mgP95 national0.0%
- MagnesiumMg²⁺1.5mgP29 national0.0%
- Iron (II)Fe²⁺0.10mgP30 national0.0%
- BariumBa²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻19,770mgP99 national100%
- BromideBr⁻65.8mgP99 national0.1%
- BicarbonateHCO₃⁻43.3mgP25 national0.1%
- SulfateSO₄²⁻11.0mgP22 national0.0%
- FluorideF⁻1.9mgP57 national0.0%
- NitrateNO₃⁻0.30mgP43 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃588mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂136mgP97 national
- Metaarsenious acidHAsO₂3.6mgP98 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂35.9mgP63 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.
- 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.