A sodium-led chloride spring — classed under the old nomenclature as pure salt water (jun-shokuen-sen) — salty and good at retaining heat. With dissolved solids around 3,304 mg/kg it is moderately mineralized, and the water carries a faint yellow-brown tint at the source. Neutral in pH and gentle on the skin, it is a low-temperature source (31.5 °C) reheated for bathing.
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
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⁺985mgP85 national75%
- Magnesiummild; a laxative if you drink itMg²⁺124mgP93 national18%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.8mgP63 national5%
- Iron (II)Fe²⁺1.1mgP64 national0.1%
- StrontiumSr²⁺1.4mgP83 national0.1%
- LithiumLi⁺0.30mgP41 national0.1%
- BariumBa²⁺0.20mgP72 national0.0%
- Potassiumminor; matters for drinking limitsK⁺32.7mgP77 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,667mgP86 national86%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻235mgP68 national9%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻147mgP52 national4%
- BromideBr⁻5.6mgP84 national0.1%
- Hydrogen phosphateHPO₄²⁻0.70mgP76 national0.0%
- FluorideF⁻0.30mgP22 national0.0%
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂21.9mgP75 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃14.6mgP2 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂61.8mgP73 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2018.
- chronic muscle or joint pain and stiffness
- 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.