A sodium chloride strong-salt spring (kyoensen) — hypertonic, neutral, and unusually mineralized at 32,209 mg/kg, well above ocean salinity. At the wellhead the water emerges yellow-brown and cloudy with a rusty-iron note and an aggressive salty bite. Pumped from a low-temperature source (27.5 degrees C) and reheated for bathing, it leaves a thick mineral coating on the skin once you climb out. Metaboric acid is high at 880.5 mg/kg, adding extra warmth retention.
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
From a certificate analyzed in 2016.
- 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⁺10,230mgP99 national78%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,306mgP97 national11%
- Magnesiummild; a laxative if you drink itMg²⁺736mgP99 national11%
- PotassiumK⁺64.7mgP88 national0.3%
- FeTotalFe13.1mgP91 national0.1%
- ManganeseMn²⁺0.40mgP61 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻18,340mgP99 national98%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻324mgP75 national1%
- BicarbonateHCO₃⁻224mgP60 national0.7%
- BromideBr⁻70.3mgP100 national0.2%
- IodideI⁻3.3mgP92 national0.0%
- FluorideF⁻0.50mgP30 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.40mgP50 national
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂881mgP100 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃15.2mgP2 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂113mgP81 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2016.
- 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.