A sodium chloride spring blended from four Surigahama wells. At 75.4 °C it ranks among Japan's hottest natural sources, neutral and isotonic, with a salty, well-mineralized body. A generous 239 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother.
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⁺3,058mgP93 national79%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺512mgP93 national15%
- Potassiumminor; matters for drinking limitsK⁺219mgP97 national3%
- Magnesiummild; a laxative if you drink itMg²⁺36.5mgP81 national2%
- LithiumLi⁺4.4mgP95 national0.4%
- StrontiumSr²⁺0.20mgP44 national0.1%
- ManganeseMn²⁺0.20mgP45 national0.0%
- AmmoniumNH₄⁺1.2mgP68 national
- BariumBa²⁺0.10mgP53 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,407mgP93 national94%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻353mgP79 national5%
- FluorideF⁻0.40mgP31 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻112mgP45 national
- Bromidetrace, sea-originBr⁻20.1mgP93 national
- Thiosulfateoxidized sulfurS₂O₃²⁻2.4mgP87 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃239mgP93 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂26.9mgP78 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.7mgP51 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.