Chikoin Onsen rises in Zentsuji as a simple, weakly radioactive cold mineral spring. Its 165.8 Bq/kg of radon places it squarely in the radon-spring family associated with the "hormesis" tradition, and comfortably past the level at which a spring is classed as therapeutically radioactive. Lightly mineralized at just 0.31 g/kg of dissolved solids and mildly alkaline, it is gentle on the body, while 64.1 mg/kg of metasilicic acid lends it a touch of Japan's beauty-water character. The source sits below 25 °C — a cold mineral spring — so it is heated before bathing.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.0mgP52 national60%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺19.3mgP4 national25%
- Magnesiummild; a laxative if you drink itMg²⁺5.2mgP51 national13%
- Potassiumminor; matters for drinking limitsK⁺1.7mgP19 national1%
- Iron (II)Fe²⁺0.20mgP38 national0.3%
- LithiumLi⁺0.04mgP15 national0.3%
- StrontiumSr²⁺0.09mgP15 national
- Manganesetrace; companion to iron springsMn²⁺0.04mgP20 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻166mgP54 national85%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻6.8mgP9 national6%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻7.7mgP18 national5%
- Fluoridetrace; drinking limits applyF⁻2.5mgP64 national4%
- Bromidetrace, sea-originBr⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃64.1mgP39 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.20mgP6 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂15.1mgP47 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
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.