A blend of six Shima Onsen sources — Shinkoku-no-yu, Tsubame-no-yu, Daisan-no-yu, Bosatsu-no-yu, Kakashibashishita-no-yu and Taki-no-yu — gathered in an underground storage tank before it reaches the baths. It is a sodium-calcium chloride-sulfate water: weakly alkaline and soft on the skin, moderately mineralized at 1,490 mg/kg dissolved solids, with the salty, heat-retaining character of a chloride spring and sulfate adding quiet body. Metasilicic acid runs to 105 mg/kg, enough to leave the thin silica film that places this water in Japan's beauty-water family. The sample was taken at a bath inlet rather than the wellhead; the posted disclosure states the tubs are neither recirculated nor chlorinated — the water is drained and refilled daily — with plain water added only when the bath runs too hot.
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⁺285mgP60 national59%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺161mgP82 national38%
- Potassiumminor; matters for drinking limitsK⁺21.2mgP65 national3%
- MagnesiumMg²⁺2.3mgP35 national0.9%
- ManganeseMn²⁺0.13mgP36 national0.0%
- Iron (II)Fe²⁺0.04mgP17 national0.0%
- AluminumAl³⁺< —mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻492mgP74 national65%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻299mgP73 national29%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻79.4mgP39 national6%
- BromideBr⁻3.2mgP77 national0.2%
- FluorideF⁻0.50mgP32 national0.1%
- IodideI⁻1.3mgP83 national0.1%
- CarbonateCO₃²⁻0.10mgP11 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃105mgP56 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂35.1mgP82 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.0mgP94 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.