A sodium chloride spring sampled at the wellhead at 28.5 C — a cool-mineral source heated for bathing. Weakly alkaline at pH 8.1 with a soft feel on the skin, and a very faintly salty note on tasting. Moderately mineralized at 1,719 mg/kg total dissolved solids, with enough body to leave noticeable warmth after a soak.
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⁺601mgP79 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.9mgP32 national3%
- Potassiumminor; matters for drinking limitsK⁺15.3mgP53 national1%
- Magnesiummild; a laxative if you drink itMg²⁺3.5mgP41 national1%
- FeTotalFe0.70mgP67 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —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⁻857mgP80 national89%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻134mgP47 national8%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻12.0mgP56 national1%
- Fluoridetrace; drinking limits applyF⁻6.0mgP82 national1%
- SulfateSO₄²⁻1.7mgP9 national0.1%
- Hydrogen sulfideHS⁻—mg0.1%
- ThiosulfateS₂O₃²⁻0.20mgP55 national0.0%
- BO2BO₂⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂53.1mgP89 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃18.2mgP4 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- 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.
- 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.