A sodium chloride strong-salt spring with dissolved solids of 25.5 g/kg — richly mineralized water that leaves a noticeable salt film on the skin. Weakly alkaline and hypertonic, it runs colorless and clear at the source with a slightly salty, faintly metallic tang. The chloride character makes it strongly heat-retaining, so warmth lingers well after 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, nettō 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
- as listed in Appendix Table 5 of the analysis report
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⁺9,096mgP99 national91%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺313mgP89 national4%
- Magnesiummild; a laxative if you drink itMg²⁺123mgP93 national2%
- StrontiumSr²⁺113mgP100 national0.6%
- LithiumLi⁺17.6mgP99 national0.6%
- PotassiumK⁺91.9mgP91 national0.5%
- BariumBa²⁺159mgP100 national0.5%
- AmmoniumNH₄⁺36.3mgP99 national0.5%
- Iron (III)Fe³⁺3.9mgP94 national0.1%
- Iron (II)Fe²⁺1.1mgP63 national0.0%
- ManganeseMn²⁺1.1mgP78 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- ZincZn²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻15,271mgP99 national99%
- BicarbonateHCO₃⁻210mgP58 national0.8%
- BromideBr⁻43.8mgP98 national0.1%
- IodideI⁻1.9mgP88 national0.0%
- FluorideF⁻0.40mgP31 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃22.0mgP6 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.9mgP76 national
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂51.6mgP69 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- as listed in Appendix Table 5 of the analysis report
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.