Kofuku no Yu is a blend of three separate sources: the Yueikai No. 1 borehole, the Shibu Onsen combined source, and the Kadoya source, brought together in the inn's regulating tank before the water reaches the baths. The result is a sodium-calcium chloride water with sulfate adding quiet body, weakly acidic, almost colorless and clear, with a faint salty tang on the lips. It leaves the ground above 65 °C, putting it among Japan's hottest natural sources, and carries a generous 156.5 mg/kg of metasilicic acid. That silica film places it squarely in Japan's beauty-water family and leaves skin noticeably smoother after a soak.
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, netto 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⁺225mgP57 national59%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺103mgP74 national31%
- Magnesiummild; a laxative if you drink itMg²⁺8.8mgP60 national4%
- Potassiumminor; matters for drinking limitsK⁺22.6mgP67 national3%
- Aluminumshows up in acidic volcanic waterAl³⁺2.5mgP81 national2%
- LithiumLi⁺0.60mgP59 national0.5%
- Iron (II)Fe²⁺0.40mgP49 national0.1%
- ManganeseMn²⁺0.40mgP61 national0.1%
- StrontiumSr²⁺0.40mgP61 national0.1%
- AmmoniumNH₄⁺0.04mgP12 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻296mgP65 national50%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻383mgP81 national48%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻19.5mgP13 national2%
- FluorideF⁻1.4mgP53 national0.4%
- BromideBr⁻1.5mgP68 national0.1%
- ThiosulfateS₂O₃²⁻0.30mgP57 national0.1%
- IodideI⁻0.40mgP66 national0.0%
- HSO4HSO40.08mgP38 national0.0%
- NitrateNO₃⁻0.04mgP19 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃157mgP71 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂40.1mgP85 national
- Metaarsenious acidHAsO₂0.60mgP83 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂19.6mgP50 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.