A sodium chloride spring — salty and notably heat-retaining. It is weakly alkaline and soft on the skin, and the sensory test records it as colorless, clear, tasteless and odorless. With 1,366 mg/kg of dissolved solids it is moderately mineralized, giving the water a noticeable body.
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
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⁺391mgP69 national83%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺63.4mgP62 national15%
- PotassiumK⁺4.6mgP33 national0.6%
- MagnesiumMg²⁺1.3mgP28 national0.5%
- LithiumLi⁺0.50mgP55 national0.3%
- AluminumAl³⁺0.0000mg0.0%
- BariumBa²⁺0.0000mg0.0%
- CopperCu²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- ZincZn²⁺0.0000mg0.0%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.40mgP48 national
- StrontiumSr²⁺0.40mgP62 national
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻747mgP78 national92%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻76.5mgP38 national5%
- Fluoridetrace; drinking limits applyF⁻7.6mgP85 national2%
- CarbonateCO₃²⁻1.2mgP21 national0.3%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- NitrateNO₃⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Bromidetrace, sea-originBr⁻1.6mgP68 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.40mgP4 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.20mgP40 national
- Iodideyellows in air; helps blood lipidsI⁻0.10mgP37 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃49.5mgP28 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂19.3mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂87.0mgP77 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.06mgP57 national
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.