A sodium-and-calcium chloride spring with sulfate adding quiet body, this is a moderately mineralized, neutral-pH water with noticeable substance — chloride at 2,917 mg/kg and sulfate at 1,216 mg/kg. The wellhead is among Japan's hottest natural sources at over 74 °C, and it is served as untreated source flow (no dilution, heating, recirculation, or disinfection), reaching the bath at around 41 °C. The salt-and-sulfate balance makes for a heat-retaining soak that lingers on the 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
Take care
- acute illness, especially when feverish
- active tuberculosis
- severe heart or lung disease
- renal failure with edema
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⁺1,244mgP87 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺782mgP95 national
- Magnesiummild; a laxative if you drink itMg²⁺59.0mgP86 national
- Potassiumminor; matters for drinking limitsK⁺47.2mgP83 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺2.2mgP73 national
- Manganesetrace; companion to iron springsMn²⁺0.04mgP20 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,917mgP89 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,216mgP98 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻12.0mgP7 national
- Bromidetrace, sea-originBr⁻5.9mgP83 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃84.9mgP48 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.0mgP61 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂10.1mgP41 national
Measured at the source
On the certificate
- chronic muscle or joint pain and stiffness
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.