A sodium-led bicarbonate-chloride spring drawn from 2,000 m underground in Yokohama's Aoba ward. It is the classic jūsō-sen (soda-water) style softened by a salty chloride edge, and at weakly alkaline pH it sits squarely in Japan's beauty-water family, leaving skin smooth. The water carries a yellow tint and a faint moor (peat) aroma, with moderate mineralization that gives the bath noticeable body.
Good for
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
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, nettō 熱の湯).
cold sensitivity
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 phase of illness (especially when feverish)
- Active tuberculosis
- Advanced malignant tumors, severe anemia and other serious illnesses
- Severe cardiopulmonary impairment causing breathlessness on slight exertion
- Severe kidney disease accompanied by edema
- Gastrointestinal bleeding
- When there is 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⁺375mgP64 national90%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺—mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺—mg
- Potassiumminor; matters for drinking limitsK⁺—mg
- LithiumLi⁺—mg
- Magnesiummild; a laxative if you drink itMg²⁺—mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻717mgP82 national64%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻210mgP54 national
- Fluoridetrace; drinking limits applyF⁻0.11mgP10 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃101mgP58 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂—mg
Measured at the source
On the certificate
- Chronic muscle or joint pain or stiffness
- Muscle stiffness due to motor paralysis
- Peripheral circulatory disturbance
- Reduced gastrointestinal function
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoidal pain
- Stress-related symptoms (sleep disorders, depressive states, etc.)
- Peripheral circulatory disturbance
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.
- 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.