A sodium-calcium chloride spring — salty and heat-retaining as the chloride family always is, with a softer weakly alkaline finish that keeps the skin from feeling drawn. Moderately mineralized at 3,444 mg/kg dissolved solids, it carries enough body to leave warmth lingering well after the bath. Color, taste and odor are all neutral on the certificate, so the chemistry rather than the sensory profile is what shows up in the bath.
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
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 2023.
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, severe anemia or other marked physical weakness
- Conditions that even slight bleeding would worsen
- Acute renal failure
- 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⁺782mgP83 national57%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺379mgP91 national32%
- Magnesiummild; a laxative if you drink itMg²⁺71.3mgP89 national10%
- PotassiumK⁺13.4mgP51 national0.6%
- StrontiumSr²⁺6.8mgP93 national0.3%
- AmmoniumNH₄⁺0.80mgP60 national0.1%
- BariumBa²⁺0.50mgP83 national0.0%
- AluminumAl³⁺0.10mgP56 national0.0%
- Iron (II)Fe²⁺0.10mgP29 national0.0%
- ManganeseMn²⁺0.10mgP37 national0.0%
- LithiumLi⁺0.10mgP27 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,900mgP87 national94%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻215mgP59 national6%
- SulfateSO₄²⁻11.1mgP23 national0.4%
- BromideBr⁻1.4mgP67 national0.0%
- FluorideF⁻0.20mgP16 national0.0%
- IodideI⁻0.60mgP74 national0.0%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂60.0mgP91 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.0mgP17 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂36.7mgP63 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP58 national
- CopperCu0.0005mgP31 national
- ArsenicAs< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2023.
- Chronic muscle or joint pain or stiffness
- Muscle stiffness associated with motor paralysis
- Peripheral circulation disorders
- Reduced gastrointestinal function
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- 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.
- 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.