A cold-mineral spring (< 25 °C) heated for bathing, but anything but light in feel: a richly mineralized calcium-magnesium chloride water carrying 24.78 g/kg of dissolved solids. Neutral pH keeps the touch settled, while the chloride load — approaching ocean salinity — leaves a noticeable mineral film on the skin once the bather emerges.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4,726mgP100 national52%
- Magnesiummild; a laxative if you drink itMg²⁺1,552mgP100 national28%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺2,045mgP91 national20%
- PotassiumK⁺33.9mgP77 national0.2%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- AmmoniumNH₄⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻14,880mgP98 national93%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,409mgP99 national7%
- BicarbonateHCO₃⁻49.4mgP27 national0.2%
- BromideBr⁻55.1mgP99 national0.1%
- IodideI⁻1.6mgP84 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Fluoridetrace; drinking limits applyF⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃21.9mgP6 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.1mgP20 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.9mgP35 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
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.