A sodium-led chloride-bicarbonate hot spring (46.9 °C at the wellhead). Weakly alkaline and soft on the skin, it sits in the Japanese 'beauty-water' family, with the bicarbonate side adding a touch of soda-water smoothness to the salty chloride base. Metasilicic acid at 78.8 mg/kg leaves the kind of thin silica film that makes skin feel noticeably softer afterwards.
Good for
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
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
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 2013.
- 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, 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⁺314mgP64 national93%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.5mgP35 national6%
- AmmoniumNH₄⁺2.0mgP81 national0.8%
- PotassiumK⁺3.4mgP28 national0.6%
- MagnesiumMg²⁺0.70mgP23 national0.4%
- Iron (II)Fe²⁺—mg0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻283mgP63 national58%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻330mgP67 national39%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.0mgP46 national2%
- FluorideF⁻1.1mgP48 national0.4%
- BromideBr⁻1.3mgP66 national0.1%
- SulfateSO₄²⁻0.80mgP6 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃78.8mgP46 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.9mgP23 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.5mgP20 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2013.
- 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
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Autonomic nervous system instability, stress-related symptoms (sleep disorders, depressed mood)
- Peripheral circulation disorders
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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.