A sodium-calcium bicarbonate-chloride cold mineral spring at just 9.5 °C — well below the 25 °C threshold, so it is reheated for bathing. Hypotonic and neutral in pH, with the water tinted yellow-brown and faintly turbid at the source. Moderate mineralization (1,923 mg/kg) layers bicarbonate softness over a chloride backbone that holds heat once warmed.
Good for
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
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺415mgP71 national68%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺112mgP75 national22%
- Magnesiummild; a laxative if you drink itMg²⁺28.5mgP75 national9%
- PotassiumK⁺7.5mgP40 national0.7%
- FeTotalFe0.40mgP54 national
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- CopperCu²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻885mgP87 national56%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻393mgP71 national43%
- FluorideF⁻1.9mgP57 national0.4%
- BO2BO₂⁻NDmg
- Bromidetrace, sea-originBr⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSiO3HSiO₃⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃44.3mgP25 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂31.0mgP80 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂183mgP88 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs< —mg
- CadmiumCd< —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.