Kishiro-no-Yu is a blended source — two wells, Yumoto No. 32 and Onwakai Daiichi No. 36, brought together before the water reaches the bath. The chemistry is sodium-calcium chloride with sulfate layered in for quiet body: neutral pH, moderately mineralized at 1,043 mg/kg, almost colorless and clear with neither taste nor smell. What lifts it is metasilicic acid at 128.7 mg/kg, squarely in Japan's beauty-water family — enough silica to leave a thin film that makes skin feel smoother afterwards. The sample behind this certificate was drawn at the bath inlet rather than at the wellhead.
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⁺191mgP50 national60%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺90.1mgP69 national33%
- Potassiumminor; matters for drinking limitsK⁺18.1mgP60 national3%
- Magnesiummild; a laxative if you drink itMg²⁺5.2mgP51 national3%
- LithiumLi⁺0.50mgP52 national0.5%
- StrontiumSr²⁺0.40mgP61 national0.1%
- ManganeseMn²⁺0.04mgP20 national0.0%
- AluminumAl³⁺0.01mgP18 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻270mgP62 national55%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻268mgP71 national40%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻30.5mgP19 national4%
- FluorideF⁻0.90mgP43 national0.4%
- BromideBr⁻1.3mgP66 national0.1%
- NitrateNO₃⁻1.1mgP77 national0.1%
- IodideI⁻0.30mgP61 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃129mgP65 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂34.6mgP82 national
- Metaarsenious acidHAsO₂0.60mgP83 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.1mgP35 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.