A calcium- and sodium-led sulfate–chloride spring emerging at 45.4 °C, weakly alkaline and hypotonic. Sulfate dominates the anion balance (about 90 mval%) with chloride underneath, giving the water a settled, mineral-rich body rather than a salty bite. The posted indications include cuts, peripheral circulation issues, sensitivity to cold and dry skin.
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
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, netto 熱の湯).
cold sensitivity
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
From a certificate analyzed in 2020.
- General contraindications: active stage of any illness (especially with fever), active tuberculosis, advanced malignant tumor, severe anemia or other marked physical weakness, severe heart or lung disease causing shortness of breath 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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺174mgP84 national59%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺136mgP37 national40%
- PotassiumK⁺5.4mgP35 national0.9%
- MagnesiumMg²⁺0.70mgP23 national0.4%
- StrontiumSr²⁺0.60mgP73 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻634mgP91 national90%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻27.7mgP19 national5%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻43.4mgP28 national5%
- FluorideF⁻0.40mgP28 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃40.0mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.80mgP11 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.4mgP28 national
- ArsenicAs0.02mgP30 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2020.
- General indications: chronic muscle or joint pain or stiffness (the chronic phase of rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like), muscle stiffness associated with motor paralysis, reduced gastrointestinal function (heaviness, bloating), mild hypertension, impaired glucose tolerance (diabetes), mild hypercholesterolemia, mild asthma or emphysema, hemorrhoid pain, autonomic nervous system instability, stress-related symptoms (sleep disorders, depressed mood), convalescence, fatigue recovery, general health improvement
- Spring-type indications: cuts, peripheral circulation disorders, cold sensitivity, depressed mood, dry skin
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.