A sodium-led bicarbonate spring — the classic 'jūsō-sen' style — layered with sulfate body and a salt-tinged finish, emerging hot at 59 °C. Alkaline (pH 8.6) with a slippery beauty-water feel on the skin. A generous 224 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
Good for
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
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
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
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⁺183mgP47 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺24.2mgP42 national
- Potassiumminor; matters for drinking limitsK⁺20.3mgP63 national
- Magnesiummild; a laxative if you drink itMg²⁺4.1mgP44 national
- LithiumLi⁺0.11mgP29 national
- StrontiumSr²⁺0.10mgP17 national
- BariumBa²⁺0.03mgP32 national
- Manganesetrace; companion to iron springsMn²⁺0.02mgP17 national
- ZincZn²⁺0.02mgP44 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.01mgP19 national
- FeTotalFe< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻281mgP64 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻147mgP61 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻86.7mgP37 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻27.0mgP81 national
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national
- Fluoridetrace; drinking limits applyF⁻0.10mgP9 national
- Bromidetrace, sea-originBr⁻< —mg
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃224mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.2mgP37 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.2mgP19 national
- ArsenicAs0.13mgP69 national
- CopperCu0.0020mgP42 national
- LeadPb0.0010mgP20 national
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.