Toyako Onsen's blended supply — a mixing of wells 5, 6, 9, 12 and 13 together with the community wells 1, 2, 4, 5 and 6 and the KH-1 collector — is a sodium-led chloride spring with sulfate and bicarbonate layered in for a softer finish. The water arrives at the mixing tank at a warm 50.5 °C and neutral pH, carrying a faint yellow tint and a mild salty edge at the source. Moderately mineralized (about 3.1 g/kg dissolved solids) and generously stocked with metasilicic acid at 170.8 mg/kg, it leaves the kind of silica film on the skin that puts it squarely in the Japanese 'beauty-water' family.
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⁺699mgP81 national68%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺204mgP85 national23%
- Magnesiummild; a laxative if you drink itMg²⁺36.4mgP81 national7%
- Potassiumminor; matters for drinking limitsK⁺40.3mgP81 national2%
- ManganeseMn²⁺1.9mgP90 national0.2%
- Iron (II)Fe²⁺1.8mgP70 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,169mgP82 national72%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻326mgP76 national15%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻356mgP68 national13%
- FluorideF⁻0.70mgP40 national0.1%
- BromideBr⁻2.6mgP74 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃171mgP77 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂70.9mgP93 national
- Radonfaint radon — studied for joint pain (hormesis)Rn0.20mgP18 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.
- 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.