ラRadon
A weakly radioactive spring in Saga at Kumanokawa Onsen. Alkaline at pH 9.29 with a noticeably slippery feel, sitting in Japan's beauty-water family. It emerges at around 30 degrees Celsius as a low-temperature source and is valued in the local radon-spring tradition.
Good for
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
On the certificate:ConvalescenceFatigue recoveryGeneral health improvement
Take care
Contraindications
- 急性疾患（特に熱のある場合）
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全
- Renal failure
- Bleeding disorders
- Severe anemia
- 妊娠中（特に初期と末期）
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.
Cationsshare of charge (mval%)
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺32.7mgP7 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.0mgP15 national
- Potassiumminor; matters for drinking limitsK⁺1.7mgP19 national
Anionsshare of charge (mval%)
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻75.5mgP37 national
- Fluoridetrace; drinking limits applyF⁻6.9mgP87 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻6.9mgP19 national
Measured at the source
Source temperature
30.0°C
Low temp.
Tonicity
0.15g/kg
Hypotonic
On the certificate
- 神経痛
- Muscle pain
- Joint pain
- Frozen shoulder
- Motor paralysis
- Joint stiffness
- Bruises
- Sprains
- Chronic digestive disorders
- Hemorrhoids
- 冷え症
- Chronic digestive disorders
- 高血圧症
- 胆石症
- 神経痛
- 筋肉痛
- 関節痛
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.
- 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.