A sodium-calcium chloride-sulfate spring drawn from the Furo-no-Yu source at Shima Onsen. The pH is neutral and mineralization is moderate at roughly 1.4 g/kg, pairing the heat-retaining feel of chloride water with the quiet body that sulfate adds. Metasilicic acid runs to 104 mg/kg — enough to leave the thin silica film that puts this water in Japan's beauty-water family. The posted usage disclosure lists no heating, no recirculation and no disinfection; water is added only from June to August, when the source runs too hot to serve undiluted.
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⁺276mgP60 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺167mgP83 national
- Potassiumminor; matters for drinking limitsK⁺20.0mgP62 national
- Magnesiummild; a laxative if you drink itMg²⁺2.2mgP35 national
- Manganesetrace; companion to iron springsMn²⁺0.13mgP37 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.12mgP29 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻455mgP74 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻280mgP70 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻79.4mgP40 national
- Fluoridetrace; drinking limits applyF⁻0.60mgP36 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃104mgP55 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂31.2mgP80 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂12.8mgP44 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.98mgP93 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.