A sodium-chloride spring emerging hot at 44.6 °C, weakly alkaline and soft on the skin. At the source the water carries a faint brown tint with a salty tang and a light metallic-iron note. Moderately mineralized (3,456 mg/kg dissolved solids) and unusually rich in metasilicic acid at 170 mg/kg — enough silica to leave a thin moisturizing film, which puts it squarely in 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
From a certificate analyzed in 2019.
- 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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺724mgP82 national59%
- Magnesiummild; a laxative if you drink itMg²⁺125mgP93 national19%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺160mgP83 national15%
- Potassiumminor; matters for drinking limitsK⁺89.4mgP91 national4%
- AmmoniumNH₄⁺24.1mgP98 national3%
- Iron (II)Fe²⁺2.0mgP73 national0.1%
- LithiumLi⁺0.05mgP16 national0.1%
- ManganeseMn²⁺1.1mgP79 national0.1%
- StrontiumSr²⁺1.7mgP85 national0.1%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,494mgP85 national79%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻647mgP82 national20%
- SulfateSO₄²⁻11.1mgP23 national0.4%
- Hydrogen phosphateHPO₄²⁻9.4mgP95 national0.4%
- BromideBr⁻3.2mgP78 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃170mgP77 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.5mgP25 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂35.2mgP62 national
- ArsenicAs0.0060mgP18 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0005mgP48 national
- CadmiumCd< —mg
- CopperCu< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2019.
- 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)
- 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, mildly 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.