A blend of two naturally emerging Shima Onsen wells — Iwabun-no-yu and Meiji-yu — sampled from the storage tank where they come together. Sodium and calcium lead a chloride water with sulfate layered in for quiet body: neutral pH, moderately mineralized at about 1.6 g/kg, salty enough to hold its heat well. Metasilicic acid at 106 mg/kg puts it in Japan's beauty-water family, leaving the thin 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, netto 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⁺307mgP63 national57%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺178mgP84 national38%
- Potassiumminor; matters for drinking limitsK⁺36.2mgP79 national4%
- MagnesiumMg²⁺0.94mgP25 national0.3%
- ManganeseMn²⁺0.46mgP60 national0.1%
- Iron (II)Fe²⁺0.03mgP14 national0.0%
- AluminumAl³⁺< —mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻466mgP74 national56%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻462mgP85 national41%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻29.0mgP18 national2%
- BromideBr⁻2.7mgP76 national0.1%
- FluorideF⁻0.60mgP36 national0.1%
- IodideI⁻1.3mgP84 national0.0%
- CarbonateCO₃²⁻< —mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃106mgP56 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂32.2mgP80 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂21.6mgP52 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.88mgP93 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.