A sodium-calcium spring led by sulfate with a chloride sub-type — sulfate gives quiet body, chloride adds a salty edge to the heat-retaining feel. Hot at the wellhead (50.5 °C), neutral in pH, and visually almost colourless with only a faint salty note. Moderately mineralized at 1,418 mg/kg dissolved solids, and notably rich in metasilicic acid (153.8 mg/kg) — that silica leaves a thin moisturising film that makes the water sit smoothly on the skin. A naturally welling source at Yokoyu in Yamanouchi.
Good for
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
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⁺253mgP57 national58%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺139mgP79 national37%
- Potassiumminor; matters for drinking limitsK⁺21.1mgP65 national3%
- Magnesiummild; a laxative if you drink itMg²⁺4.8mgP49 national2%
- LithiumLi⁺0.60mgP61 national0.5%
- StrontiumSr²⁺0.40mgP62 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻449mgP85 national50%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻313mgP65 national47%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻29.8mgP18 national3%
- HSiO3HSiO₃⁻0.12mgP44 national0.6%
- IodideI⁻0.40mgP66 national0.0%
- AsO2AsO₂⁻0.70mgP98 national
- NitrateNO₃⁻0.08mgP22 national
- BO2BO₂⁻NDmg
- Bromidetrace, sea-originBr⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Fluoridetrace; drinking limits applyF⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃154mgP69 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂44.8mgP86 national
- Metaarsenious acidHAsO₂0.60mgP85 national
- H2SO4FreeH2SO4FreeNDmg
- Phosphoric acidH₃PO₄NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.1mgP35 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- CadmiumCd< —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.