A sodium- and calcium-led sulfate spring with a settled, mineral-rich feel and noticeable body. Weakly alkaline and soft on the skin, the source emerges hot at 55.2 °C and is brought down to 41.0 °C for the bath. It is served without dilution, supplementary heating, or recirculation; only chlorine-based disinfection is applied to meet hygiene standards.
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
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⁺700mgP81 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺322mgP89 national
- Potassiumminor; matters for drinking limitsK⁺10.4mgP46 national
- Magnesiummild; a laxative if you drink itMg²⁺0.50mgP20 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.09mgP19 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻2,137mgP100 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻103mgP41 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻14.7mgP9 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻2.4mgP26 national
- Bromidetrace, sea-originBr⁻0.09mgP9 national
- Iodideyellows in air; helps blood lipidsI⁻0.05mgP25 national
- HydroxideOH⁻0.04mgP35 national
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.1mgP17 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.2mgP47 national
- Metaarsenious acidHAsO₂0.30mgP76 national
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.
- 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.