A sodium-led bicarbonate spring — the classic jūsō-sen style — layered with a salty chloride edge and the quiet body of sulfate, emerging hot at 51.0 °C. Weakly alkaline and soft on the skin, it sits in the Japanese beauty-water family. With 88 mg/kg of metasilicic acid, it leaves a thin silica film that makes skin feel 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
Take care
- active phase of illness (especially when feverish)
- active tuberculosis
- advanced malignant tumor or severe systemic weakness such as serious anemia
- severe heart or lung disease causing breathlessness on slight exertion
- severe kidney disease with edema
- gastrointestinal bleeding
- 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⁺521mgP74 national96%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.9mgP24 national2%
- Potassiumminor; matters for drinking limitsK⁺10.7mgP47 national1%
- MagnesiumMg²⁺2.2mgP37 national0.8%
- LithiumLi⁺0.40mgP47 national0.3%
- AmmoniumNH₄⁺0.80mgP57 national0.2%
- Iron (II)Fe²⁺0.20mgP41 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻788mgP84 national53%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻208mgP54 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻253mgP72 national22%
- FluorideF⁻1.0mgP49 national0.2%
- Bromidetrace, sea-originBr⁻0.60mgP48 national
- Hydrogen phosphateHPO₄²⁻0.10mgP43 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃88.1mgP53 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.8mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂59.3mgP71 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPb< —mg
Measured at the source
On the certificate
- osteoarthritis
- bruises
- sensitivity to cold
- weakened gastrointestinal function
- impaired glucose tolerance (diabetes)
- mild hypercholesterolemia
- mild asthma or emphysema
- stress-related symptoms (sleep disturbance, depressive state)
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.