Yoro Onsen Spring No. 2 is a sodium-calcium chloride spring — salty, heat-retaining water in which chloride accounts for almost the entire anion balance. Moderately mineralized with noticeable body, it leaves the faint mineral film that keeps warmth in long after you climb out. Unusually, the posted analysis classes it as isotonic and neutral, so its osmotic pressure sits close to the body's own and the soak feels gentler than the salt content suggests. The notice lists cuts, peripheral circulation disorders, cold sensitivity and dry skin among the conditions this spring type is documented for.
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
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
- Marked physical debilitation, such as advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- Severe kidney disease accompanied by 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⁺1,729mgP91 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,040mgP96 national36%
- Magnesiummild; a laxative if you drink itMg²⁺171mgP95 national10%
- PotassiumK⁺19.6mgP62 national0.3%
- StrontiumSr²⁺12.0mgP95 national0.2%
- BariumBa²⁺10.0mgP98 national0.1%
- AmmoniumNH₄⁺2.0mgP81 national0.1%
- Iron (II)Fe²⁺2.7mgP76 national0.1%
- ManganeseMn²⁺2.1mgP91 national0.1%
- Iron (III)Fe³⁺0.40mgP76 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,792mgP93 national94%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻476mgP76 national5%
- BromideBr⁻18.3mgP93 national0.2%
- IodideI⁻5.2mgP95 national0.0%
- SulfateSO₄²⁻1.3mgP8 national0.0%
- Hydrogen phosphateHPO₄²⁻0.50mgP70 national0.0%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃44.6mgP25 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.80mgP11 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂22.4mgP52 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.06mgP58 national
- CopperCu< —mg
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)
- Muscle stiffness associated with motor paralysis
- 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)
- Peripheral circulation disorders
- Dry skin
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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.