The in-house source at Yumura Hotel in Kofu's Yumura Onsen district — a hot (45.8 °C) sodium-calcium chloride-sulfate spring. Alkaline pH 8.5 gives the slippery skin feel of Japan's beauty-water family, while chloride salinity and sulfate body add palpable weight. At the wellhead the water carries a slight milky cloudiness and a faint salty taste. Moderate mineralization at 1,781 mg/kg keeps it well-rounded — long-retained warmth paired with soft skin contact.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺420mgP71 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺120mgP77 national23%
- ZincZn²⁺38.7mgP100 national5%
- PotassiumK⁺8.2mgP41 national0.8%
- LithiumLi⁺0.60mgP61 national0.3%
- StrontiumSr²⁺0.80mgP76 national0.1%
- Iron (II)Fe²⁺0.20mgP39 national0.0%
- AluminumAl³⁺0.10mgP54 national0.0%
- AmmoniumNH₄⁺0.10mgP24 national0.0%
- BariumBa²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- MagnesiumMg²⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻508mgP75 national55%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻498mgP87 national40%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻56.3mgP30 national4%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻11.8mgP52 national1%
- FluorideF⁻3.2mgP67 national0.7%
- BromideBr⁻1.3mgP65 national0.1%
- Hydrogen sulfideHS⁻0.50mgP57 national0.1%
- AsO2AsO₂⁻0.0000mg0.0%
- BO2BO₂⁻0.0000mg0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- HSiO3HSiO₃⁻0.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃95.7mgP54 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.1mgP72 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.