A sodium-calcium chloride-sulfate spring blended from three Yokoyu wells. Weakly alkaline and soft on the skin, the water is almost colorless and transparent yet carries a faint salty, sulfurous edge. The source emerges scaldingly hot at 86.5 °C, among Japan's hottest, and its 199 mg/kg of metasilicic acid leaves a thin silica film for noticeably smoother skin.
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⁺335mgP64 national69%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺112mgP75 national26%
- Potassiumminor; matters for drinking limitsK⁺30.0mgP74 national4%
- LithiumLi⁺0.90mgP72 national0.6%
- HydrogenH⁺0.0000mg0.6%
- Aluminumshows up in acidic volcanic waterAl³⁺0.04mgP27 national
- Magnesiummild; a laxative if you drink itMg²⁺0.04mgP5 national
- Manganesetrace; companion to iron springsMn²⁺0.04mgP20 national
- StrontiumSr²⁺0.04mgP14 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻448mgP72 national60%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻384mgP80 national36%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻47.7mgP27 national4%
- FluorideF⁻1.4mgP53 national0.3%
- BromideBr⁻2.2mgP71 national0.1%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.60mgP59 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃199mgP85 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.60mgP10 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.3mgP23 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.07mgP53 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.