塩Chloride
A sodium chloride spring at 38.6 °C and pH 7.7 — salty, heat-retaining water in the chloride family.
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.
Cationsshare of charge (mval%)
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺—mg
- Potassiumminor; matters for drinking limitsK⁺—mg
- LithiumLi⁺—mg
- Magnesiummild; a laxative if you drink itMg²⁺—mg
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺—mg
Anionsshare of charge (mval%)
- Bromidetrace, sea-originBr⁻—mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻—mg
- Fluoridetrace; drinking limits applyF⁻—mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻—mg
- Iodideyellows in air; helps blood lipidsI⁻—mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻—mg
Non-dissociatedmeasured by mass
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃—mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂—mg
Dissolved gasesmeasured by mass
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂—mg
Measured at the source
Source temperature
38.6°C
Warm
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.