A sodium-calcium sulfate-chloride spring drawn very hot at 83.4 °C — among Japan's hottest natural sources — carrying a faint sulfur note and a slightly bitter taste straight from the wellhead. Neutral in pH and moderately mineralized at 1,346 mg/kg, it has noticeable body in the water. A generous 189 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel smoother afterwards.
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
Take care
- acute phase of illness (especially with fever)
- active tuberculosis
- severe heart disease
- respiratory failure
- renal failure
- hemorrhagic disorders
- severe anemia
- any other illness in a progressive phase
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⁺209mgP53 national57%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺109mgP74 national34%
- Potassiumminor; matters for drinking limitsK⁺31.3mgP76 national5%
- LithiumLi⁺0.70mgP64 national
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP14 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻384mgP70 national54%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻382mgP80 national41%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻38.4mgP23 national3%
- FluorideF⁻1.7mgP55 national0.5%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃189mgP82 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.3mgP30 national
- ArsenicAs0.81mgP92 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- LeadPbNDmg
Measured at the source
On the certificate
- chronic muscle or joint pain or stiffness
- weak gastrointestinal function
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.