A blended source that brings together the Yufu-no-yu and Asagiri-no-yu wells, classified as a simple thermal spring — neutral, hypotonic and light on the body at 832 mg/kg of dissolved solids. At 71.6 °C it ranks among Japan's hottest natural sources, and the posted panel records it reaching the bathing position at 43.8–50.4 °C. What lifts it above ordinary simple water is metasilicic acid at 226 mg/kg, enough to leave the thin silica film that places this spring in Japan's beauty-water family. The panel documents the water for neuralgia, joint stiffness, bruises and sprains, convalescence and fatigue recovery.
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 2009.
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 tumor
- severe heart disease
- respiratory failure
- renal failure
- bleeding disorders
- severe anemia
- any other illness in a progressive stage
- pregnancy (especially the 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⁺156mgP41 national84%
- Potassiumminor; matters for drinking limitsK⁺31.5mgP77 national10%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺5.7mgP18 national3%
- Magnesiummild; a laxative if you drink itMg²⁺2.1mgP36 national2%
- AmmoniumNH₄⁺0.80mgP60 national0.5%
- LithiumLi⁺0.30mgP41 national0.5%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻264mgP62 national54%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻106mgP43 national37%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻32.6mgP36 national8%
- Hydrogen phosphateHPO₄²⁻0.80mgP78 national0.3%
- FluorideF⁻0.30mgP22 national0.3%
- BromideBr⁻0.20mgP28 national0.0%
- ThiosulfateS₂O₃²⁻0.20mgP52 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃226mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.2mgP36 national
- Metaarsenious acidHAsO₂0.10mgP53 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.5mgP34 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.09mgP64 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2009.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate.
- chronic digestive disorders
- hemorrhoids
- cold sensitivity
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.