A sodium chloride spring — salty, heat-retaining water that the posted notice still labels by its old name, jun-shokuen-sen ("pure salt spring"). It is weakly alkaline and soft on the skin, and a generous 134 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family; dissolved solids of 1,321 mg/kg give the water moderate body without heaviness. The notice lists cuts, peripheral circulation disorders, cold sensitivity, depressed mood and dry skin among the spring-type indications. The bath is fed by a blend of two registered wells (Onsen-mura No. 28 and No. 29).
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
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, severe anemia, or other marked physical debilitation
- Severe heart or kidney disease causing shortness of breath on slight exertion
- 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⁺341mgP66 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.5mgP52 national
- Potassiumminor; matters for drinking limitsK⁺26.6mgP72 national
- Magnesiummild; a laxative if you drink itMg²⁺3.5mgP41 national
- LithiumLi⁺0.51mgP54 national
- StrontiumSr²⁺0.13mgP25 national
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.02mgP22 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.0000mg
- ZincZn²⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻545mgP76 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻144mgP51 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻60.7mgP44 national
- HSiO3HSiO₃⁻1.7mgP52 national
- Bromidetrace, sea-originBr⁻1.1mgP59 national
- BO2BO₂⁻0.76mgP54 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.70mgP17 national
- NitrateNO₃⁻0.37mgP46 national
- Fluoridetrace; drinking limits applyF⁻0.33mgP23 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃134mgP66 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂19.5mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.9mgP25 national
- ArsenicAs0.40mgP90 national
- CadmiumCd0.0000mg
- CopperCu0.0000mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.0000mg
- LeadPb0.0000mg
Measured at the source
On the certificate
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders, depressed mood)
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.