岩根の湯Shima Onsen Iwane no Yu
A sodium-calcium chloride spring rising unaided at Shima, with sulfate adding quiet body to the water. The pH is neutral and the mineralization moderate at 1,880 mg/kg, so the water has noticeable substance without any sharpness. A generous 133 mg/kg of metasilicic acid puts it in Japan's beauty-water family: the silica leaves a thin film that makes skin feel smoother once you are out of the bath.
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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⁺445mgP73 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺126mgP77 national23%
- Potassiumminor; matters for drinking limitsK⁺54.9mgP85 national5%
- MagnesiumMg²⁺1.8mgP31 national0.5%
- Iron (II)Fe²⁺0.80mgP59 national0.1%
- ManganeseMn²⁺0.77mgP72 national0.1%
- AluminumAl³⁺< —mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻709mgP78 national74%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻307mgP74 national24%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻40.0mgP24 national2%
- BromideBr⁻4.6mgP79 national0.2%
- FluorideF⁻0.70mgP40 national0.1%
- CarbonateCO₃²⁻< —mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃133mgP66 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂54.0mgP90 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂24.2mgP55 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.5mgP96 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.