A sodium-led chloride-sulfate spring emerging at a striking 68.6 °C — among Japan's hotter natural sources. Weakly alkaline (pH 7.7) and softly mineralised at 1,307 mg/kg, it bathes clear and colourless with a faint salty tang and a whisper of H₂S on the nose. The chloride backbone carries heat well, while the sulfate sub-type adds a quiet sense of body.
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⁺340mgP65 national78%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺67.8mgP63 national18%
- Potassiumminor; matters for drinking limitsK⁺19.3mgP61 national3%
- Magnesiummild; a laxative if you drink itMg²⁺3.5mgP41 national2%
- AmmoniumNH₄⁺0.04mgP13 national0.1%
- ManganeseMn²⁺0.01mgP17 national0.1%
- FeTotalFe0.05mgP28 national0.0%
- LithiumLi⁺0.05mgP17 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻401mgP71 national60%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻269mgP71 national30%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻103mgP43 national9%
- Fluoridetrace; drinking limits applyF⁻4.6mgP77 national1%
- Bromidetrace, sea-originBr⁻0.85mgP55 national
- Iodideyellows in air; helps blood lipidsI⁻0.05mgP25 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃74.2mgP45 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂22.1mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂21.4mgP52 national
- ArsenicAs0.04mgP46 national
- CadmiumCd< —mg
- CopperCu< —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.