A simple thermal spring drawn from the Unazuki Onsen wellhead, which emerges at a striking 98.3 °C and is cooled to about 46 °C before reaching the bath. With only 402 mg/kg of dissolved solids it sits on the gentle, lightly mineralized end of the spectrum, and its weakly alkaline pH gives a soft, unprickly feel on the skin. The wellhead temperature alone places it among Japan's hottest natural sources.
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⁺99.8mgP29 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.3mgP29 national
- Potassiumminor; matters for drinking limitsK⁺5.0mgP34 national
- Magnesiummild; a laxative if you drink itMg²⁺0.83mgP24 national
- LithiumLi⁺0.49mgP51 national
- AmmoniumNH₄⁺0.27mgP34 national
- StrontiumSr²⁺0.19mgP30 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻132mgP47 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻77.9mgP34 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻26.9mgP34 national
- Fluoridetrace; drinking limits applyF⁻6.2mgP82 national
- HSiO3HSiO₃⁻0.39mgP46 national
- BO2BO₂⁻0.33mgP47 national
- Bromidetrace, sea-originBr⁻0.23mgP30 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.5mgP34 national
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.