A naturally welling radon spring at Yunoyama Onsen, carrying about 137 Bq/kg of radon — enough to place it in the radon-spring tradition associated with the 'hormesis' idea. At pH 8.0 it is weakly alkaline and soft on the skin, and the water is almost colorless, tasteless and odorless. Lightly mineralized overall, it is documented for gout, chronic digestive complaints and neuralgia.
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
Take care
From a certificate analyzed in 2004.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were removed from the official list of contraindications then, or include a condition that was. If you have a health condition, ask a doctor before bathing.
- acute illness (especially with fever)
- active tuberculosis
- malignant tumors
- severe heart disease
- respiratory failure
- renal failure
- hemorrhagic conditions
- severe anemia
- other generally progressing diseases
- pregnancy (especially early and late stages) (dropped from the official list in 2014)
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⁺37.9mgP8 national89%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.3mgP12 national9%
- Potassiumminor; matters for drinking limitsK⁺0.10mgP1 national2%
- Magnesiummild; a laxative if you drink itMg²⁺0.10mgP11 national
- LithiumLi⁺0.05mgP15 national
- StrontiumSr²⁺0.04mgP12 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0070mgP16 national
- BariumBa²⁺0.0040mgP26 national
- CopperCu²⁺0.0030mgP50 national
- FeTotalFeNDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻59.8mgP35 national55%
- Fluoridetrace; drinking limits applyF⁻8.4mgP87 national25%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8.4mgP11 national13%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻5.5mgP16 national6%
- NitrateNO₃⁻0.10mgP26 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃27.6mgP10 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.2mgP20 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.0mgP18 national
- CadmiumCdNDmg
- LeadPbNDmg
Measured at the source
On the certificate
From a certificate analyzed in 2004.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were dropped from the official list then.
- gout
- chronic digestive disorders
- chronic gallbladder disease
- gallstones (dropped from the official list in 2014)
- chronic skin conditions (dropped from the official list in 2014)
- chronic gynecological disorders (dropped from the official list in 2014)
- chronic digestive disorders
- hemorrhoids
- sensitivity to cold
- gout
- chronic digestive disorders
- chronic gallbladder disease
- gallstones
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.