Iwodani No. 12 is a simple acidic thermal spring, sharply acidic at pH 3.0 at the wellhead so it feels faintly prickly on the skin, and classified as a hot spring at a wellhead temperature of 61.9 °C. It is unusually rich in metasilicic acid at 232.9 mg/kg, which leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards. The water is colorless and transparent but carries a metallic taste and a faint metallic odor.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
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⁺37.1mgP8 national29%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺1.1mgP67 national20%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺15.4mgP33 national14%
- Potassiumminor; matters for drinking limitsK⁺29.8mgP74 national14%
- Magnesiummild; a laxative if you drink itMg²⁺8.8mgP60 national13%
- Aluminumshows up in acidic volcanic waterAl³⁺2.1mgP81 national4%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺4.3mgP79 national3%
- AmmoniumNH₄⁺2.7mgP85 national3%
- LithiumLi⁺0.30mgP44 national0.7%
- ManganeseMn²⁺0.20mgP45 national0.2%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻238mgP69 national88%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻19.9mgP17 national10%
- HSO4HSO49.0mgP57 national2%
- FluorideF⁻0.90mgP45 national0.9%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃233mgP92 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.3mgP59 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.2mgP22 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.
- 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.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official