A sodium-led sulfate-bicarbonate spring (legacy name 'soda-glauber' style), pairing the settled body of sulfate water with bicarbonate softness. Weakly alkaline, it sits in the Japanese beauty-water family. The source emerges at a scalding 98.8 °C — among Japan's hottest natural sources — and is diluted with water before reaching the bath. A generous 355 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother.
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
Take care
- active phase of illness (especially with fever)
- active tuberculosis
- advanced malignant tumors or severe physical debilitation such as marked anemia
- serious heart or lung disease causing breathlessness on slight exertion
- serious kidney disease accompanied by edema
- gastrointestinal bleeding
- any visible bleeding
- acute exacerbation of a chronic illness
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⁺381mgP69 national73%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺67.6mgP63 national15%
- Magnesiummild; a laxative if you drink itMg²⁺26.5mgP74 national10%
- Potassiumminor; matters for drinking limitsK⁺18.8mgP60 national2%
- AmmoniumNH₄⁺0.20mgP30 national0.0%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.40mgP49 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.20mgP58 national
- StrontiumSr²⁺0.20mgP42 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP53 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- ZincZn²⁺< 0.10mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻580mgP89 national52%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻406mgP72 national29%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻152mgP49 national19%
- FluorideF⁻0.40mgP29 national0.1%
- H2PO4H₂PO₄⁻0.40mgP85 national
- Bromidetrace, sea-originBr⁻0.20mgP29 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃355mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂36.4mgP83 national
- Metaarsenious acidHAsO₂0.10mgP53 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂106mgP81 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< 0.10mg
Measured at the source
On the certificate
- chronic muscle or joint pain or stiffness
- poor circulation (sensitivity to cold)
- weakened gastrointestinal function
- impaired glucose tolerance (diabetes)
- mild hypercholesterolemia
- mild asthma or emphysema
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.