A cold-mineral spring (13.3 °C) of mixed sodium-calcium chloride and bicarbonate type, carrying dissolved carbon dioxide. Neutral in pH and moderately mineralized at around 2,004 mg/kg, it is reheated to about 42 °C for bathing. The chloride edge gives a faint salty body, while the bicarbonate side softens the feel; a small iron load (~17 mg/kg) adds a quiet metallic accent.
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⁺469mgP75 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺219mgP86 national
- Magnesiummild; a laxative if you drink itMg²⁺68.4mgP88 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺17.2mgP93 national
- Manganesetrace; companion to iron springsMn²⁺4.0mgP97 national
- LithiumLi⁺3.6mgP93 national
- Fluoridetrace; drinking limits applyF⁻0.30mgP24 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂37.0mgP83 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃30.2mgP13 national
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.