Iwaya Onsen is a simple weak-radioactive spring, and at 142 Bq/kg of radon it sits squarely in the hormesis tradition that gives radon waters their following. It is a cold-mineral source (below 25 °C), so it is heated for bathing. The water is lightly mineralized and mildly alkaline — gentle and soft on the skin, colorless and clear, with a very faint hydrogen sulfide note at the wellhead. It also carries a generous load of metasilicic acid, the kind that leaves a thin moisturizing film and places it in Japan's beauty-water family.
Good for
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
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⁺183mgP47 national86%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.5mgP29 national7%
- Magnesiummild; a laxative if you drink itMg²⁺6.0mgP55 national5%
- Potassiumminor; matters for drinking limitsK⁺5.2mgP34 national1%
- LithiumLi⁺0.32mgP42 national0.5%
- Iron (II)Fe²⁺0.13mgP29 national0.1%
- StrontiumSr²⁺0.15mgP25 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- AmmoniumNH₄⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻426mgP73 national80%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻38.7mgP24 national13%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻23.7mgP32 national6%
- Fluoridetrace; drinking limits applyF⁻1.7mgP57 national1%
- CarbonateCO₃²⁻2.1mgP23 national0.8%
- BromideBr⁻0.19mgP17 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃71.8mgP44 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.8mgP54 national
- Metaarsenious acidHAsO₂0.33mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂10.8mgP41 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.