A sodium-calcium chloride spring rising from beneath the Kyoto basin. Richly mineralized at 11.6 g/kg of dissolved solids, it leaves a noticeable mineral film on the skin and falls in the hypertonic, heat-retaining 'netsu-no-yu' tradition. At the wellhead the water carries a faint yellow-brown tint with a slight iron tang and mineral note.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺3,212mgP94 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺912mgP95 national23%
- Magnesiummild; a laxative if you drink itMg²⁺80.5mgP90 national3%
- PotassiumK⁺41.6mgP81 national0.5%
- StrontiumSr²⁺34.0mgP99 national0.4%
- Iron (II)Fe²⁺5.2mgP81 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻7,065mgP95 national100%
- BicarbonateHCO₃⁻73.5mgP36 national0.6%
- BromideBr⁻20.3mgP93 national0.1%
- Iodideyellows in air; helps blood lipidsI⁻0.70mgP75 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂75.2mgP94 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃17.7mgP4 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂34.7mgP63 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.