生明湯Ikumei-yu Spring
A sodium chloride spring of hypertonic, near-neutral character that emerges at 66.8 °C — among Japan's hotter natural sources. Mineralization of 18,180 mg/kg gives it a pronounced salty body that leaves a mineral film on the skin. A respectable 106 mg/kg of metasilicic acid layers in a touch of silica smoothness on top of 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⁺5,658mgP97 national80%
- Magnesiummild; a laxative if you drink itMg²⁺546mgP99 national15%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺213mgP86 national3%
- Potassiumminor; matters for drinking limitsK⁺263mgP98 national2%
- ManganeseMn²⁺1.2mgP80 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8,713mgP96 national84%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,576mgP94 national9%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,069mgP96 national8%
- FluorideF⁻1.2mgP50 national0.0%
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃106mgP56 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂39.8mgP85 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂132mgP84 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CopperCu0.01mgP76 national
- ArsenicAs0.0010mgP6 national
- CadmiumCd< —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.