A blended source drawing on wells No. 1 through No. 5 at Iide Onsen, sampled from the storage tank where they pool together. It is a sodium-calcium chloride water with bicarbonate softness and the quiet body of sulfate layered in — neutral in pH, moderately mineralized, and carrying a generous 129 mg/kg of metasilicic acid that places it in Japan's beauty-water family and leaves skin noticeably smoother. The posted notice names cuts, peripheral circulation disorders, cold sensitivity and dry skin among the conditions this spring type is documented for. The water rises at 51.7 °C, though the same notice states that in winter, when the source cools, it may be reheated to a comfortable bathing temperature.
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
Take care
- Active stage of any illness (especially with fever)
- A debilitated condition such as active tuberculosis, advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- Severe kidney disease accompanied by edema
- Gastrointestinal bleeding
- Any visible bleeding
- Acute exacerbation of a chronic illness
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⁺249mgP57 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺182mgP84 national
- Potassiumminor; matters for drinking limitsK⁺25.6mgP70 national
- Magnesiummild; a laxative if you drink itMg²⁺5.0mgP50 national
- LithiumLi⁺0.80mgP70 national
- Manganesetrace; companion to iron springsMn²⁺0.20mgP45 national
- ZincZn²⁺0.01mgP41 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺—mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻402mgP71 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻310mgP74 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻281mgP61 national
- Fluoridetrace; drinking limits applyF⁻2.8mgP64 national
- Bromidetrace, sea-originBr⁻1.0mgP59 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.10mgP11 national
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃129mgP65 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.7mgP40 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂161mgP86 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S—mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Reduced gastrointestinal function (heaviness, bloating)
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Stress-related symptoms (sleep disorders, depressed mood)
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.