礒崎2号Isozaki Spring #2
A high-temperature sulfur-bearing sodium chloride strong-salt spring (74 °C at source) from Wakayama's Shirahama coast. Strongly mineralized at roughly 20 g/kg dissolved solids, it leaves the distinct mineral coating of a 'kyō-en-sen' on the skin. Classified as sulfur-bearing ('sulfur-bearing') thanks to trace H₂S, and carries a fair load of metasilicic acid (79.5 mg/kg) for a softer skin-feel beneath the salt.
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⁺6,294mgP98 national79%
- Magnesiummild; a laxative if you drink itMg²⁺667mgP99 national16%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺168mgP83 national2%
- Potassiumminor; matters for drinking limitsK⁺242mgP98 national2%
- ManganeseMn²⁺1.3mgP83 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10,050mgP97 national88%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,868mgP95 national9%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻405mgP82 national3%
- FluorideF⁻2.7mgP64 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.20mgP40 national
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSiO3HSiO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃79.5mgP47 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂58.7mgP91 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂299mgP92 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.2mgP76 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.