A sodium chloride spring rising at 99.3 °C — one of Japan's hottest natural sources, requiring cooling before bathing. The water is almost colorless and clear at the wellhead, with a faintly salty edge and a whisper of hydrogen sulfide. Weakly alkaline (pH 8.4), it sits in the classic Japanese 'beauty-water' family, and a generous 208 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺674mgP81 national82%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺91.2mgP70 national13%
- Potassiumminor; matters for drinking limitsK⁺68.2mgP89 national5%
- LithiumLi⁺0.60mgP61 national0.3%
- StrontiumSr²⁺0.80mgP76 national0.1%
- MagnesiumMg²⁺0.20mgP14 national0.0%
- AmmoniumNH₄⁺0.10mgP24 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,111mgP81 national88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻163mgP61 national9%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻22.7mgP71 national2%
- BicarbonateHCO₃⁻8.5mgP5 national0.4%
- FluorideF⁻1.5mgP54 national0.2%
- BromideBr⁻2.4mgP72 national0.1%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- HydroxideOH⁻0.04mgP35 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃208mgP87 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.0mgP42 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- 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.