A sodium chloride spring with a layer of bicarbonate softness — 44 °C at the wellhead, hypotonic and weakly alkaline. The salt edge sits on top of a smoothing 'juso-sen' (bicarbonate) base, gentle on the skin rather than aggressive. Metasilicic acid at 53.8 mg/kg places it just into the Japanese 'beauty-water' family.
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
- Acute illness (especially when fever is present)
- 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
- Active phase 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⁺620mgP80 national73%
- Magnesiummild; a laxative if you drink itMg²⁺62.5mgP88 national14%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺91.9mgP71 national12%
- PotassiumK⁺9.6mgP45 national0.7%
- Iron (II)Fe²⁺1.9mgP72 national0.2%
- Manganesetrace; companion to iron springsMn²⁺0.08mgP25 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0080mgP16 national
- ZincZn²⁺0.0010mgP32 national
- LithiumLi⁺NDmg
- AmmoniumNH₄⁺NDmg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻982mgP81 national75%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻455mgP75 national20%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻92.4mgP53 national5%
- CarbonateCO₃²⁻1.1mgP17 national0.1%
- FluorideF⁻0.50mgP30 national0.1%
- Hydrogen sulfideHS⁻NDmg0.0%
- ThiosulfateS₂O₃²⁻NDmg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃53.8mgP32 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂10.6mgP57 national
- Metaarsenious acidHAsO₂0.20mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂145mgP84 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
On the certificate
- Chronic digestive disorders
- Hemorrhoids
- 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.