A sodium-led strong-salt chloride spring in which chloride makes up 98 percent of the anions, giving a dense, salty, heat-retaining water. It is hypertonic and richly mineralized, leaving a mineral film on the skin after bathing. Colorless and clear at the wellhead, it clears to a pale yellow once exposed to air and throws a reddish-brown iron sediment, with a distinctly salty, metallic taste on the lips.
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
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⁺6,000mgP98 national83%
- Magnesiummild; a laxative if you drink itMg²⁺326mgP98 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺516mgP93 national8%
- AmmoniumNH₄⁺15.8mgP97 national0.3%
- StrontiumSr²⁺20.9mgP96 national0.1%
- PotassiumK⁺18.7mgP60 national0.1%
- LithiumLi⁺1.7mgP89 national0.1%
- Iron (II)Fe²⁺5.6mgP82 national0.1%
- BariumBa²⁺3.6mgP95 national0.0%
- ManganeseMn²⁺0.40mgP59 national0.0%
- AluminumAl³⁺0.20mgP64 national0.0%
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10,900mgP97 national98%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻343mgP67 national2%
- BromideBr⁻39.4mgP98 national0.2%
- FluorideF⁻0.90mgP45 national0.0%
- IodideI⁻3.3mgP91 national0.0%
- Hydrogen phosphateHPO₄²⁻1.5mgP86 national0.0%
- ThiosulfateS₂O₃²⁻< —mg0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- NitrateNO₃⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂318mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃30.5mgP13 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂72.6mgP74 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CopperCu0.0060mgP53 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- 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.