An iron-bearing sodium-calcium chloride spring classified as a strong-salt water (kyoensen). Total dissolved solids reach 23.7 g/kg, in the hypersaline band that approaches ocean salinity, and although the water emerges colorless and clear it deposits a brown iron-oxide sediment if left to stand. The taste is unmistakably salty and bitter with a metallic edge from 48.7 mg/kg of iron, and an unusual 834 mg/kg of free carbon dioxide gives it a fine prickle on entry.
Good for
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
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⁺6,370mgP98 national69%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,060mgP98 national26%
- Magnesiummild; a laxative if you drink itMg²⁺144mgP94 national3%
- Potassiumminor; matters for drinking limitsK⁺229mgP98 national1%
- LithiumLi⁺23.7mgP99 national0.8%
- Iron (II)Fe²⁺48.7mgP98 national0.4%
- StrontiumSr²⁺33.7mgP99 national0.2%
- BariumBa²⁺8.1mgP98 national0.0%
- ManganeseMn²⁺1.7mgP88 national0.0%
- AluminumAl³⁺0.14mgP55 national0.0%
- ZincZn²⁺0.32mgP87 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻14,200mgP98 national99%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻300mgP65 national1%
- BromideBr⁻35.8mgP95 national0.1%
- FluorideF⁻3.2mgP66 national0.0%
- IodideI⁻0.15mgP38 national0.0%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂168mgP98 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃41.5mgP22 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂834mgP98 national
- ArsenicAs0.07mgP59 national
- CopperCu0.07mgP98 national
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.