A calcium- and sodium-led sodium chloride spring — salty and heat-retaining, leaving a faint mineral film on the skin. Richly mineralized at 9,132 mg/kg of dissolved solids, it is isotonic and neutral in pH, so it sits gently against the body. The bath water is diluted, reheated, recirculated, and chlorine-disinfected before reaching the pool, and the certificate marks it as bathing-only, not for drinking.
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, 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 or marked physical debilitation such as severe anemia
- Severe heart or lung disease causing breathlessness 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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,042mgP98 national63%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺1,327mgP88 national36%
- PotassiumK⁺22.3mgP67 national0.3%
- MagnesiumMg²⁺5.8mgP54 national0.3%
- Iron (II)Fe²⁺0.10mgP30 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,694mgP94 national100%
- BicarbonateHCO₃⁻13.7mgP9 national0.1%
- FluorideF⁻1.9mgP57 national0.1%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃23.3mgP8 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.50mgP8 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.2mgP22 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic pain or stiffness of muscles or joints (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains in their chronic phase)
- Muscle stiffness associated with motor paralysis
- Weakened gastrointestinal function (heaviness in the stomach, gas in the intestines)
- Mild hypertension
- Impaired glucose tolerance (diabetes mellitus)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoidal pain
- Stress-related symptoms (sleep disturbance, depressive states)
- Peripheral circulatory 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.