A sodium-calcium-magnesium chloride strong-salt spring (formerly classified in the old saline-iron style). With 17.1 g/kg of dissolved solids it is richly mineralized and hypertonic, leaving a mineral film on the skin. It is iron-rich at 50 mg/kg, oxidizing to a brown tint with a characteristic sediment and carrying a faint metallic note. It is also an unusual carbonated spring, with free CO₂ at 2,631 mg/kg producing fine bubbles that cling to the skin. The source emerges cool at 25 °C, so it is reheated for bathing via a heat exchanger.
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
Iron
Iron springs supply iron, traditionally taken as a drink for iron-deficiency anemia where drinking is permitted.
anemia (drinking)
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
Take care
- acute phase of illness (especially when feverish)
- active tuberculosis
- advanced malignant tumors
- severe heart or lung disease
- kidney failure with edema
- gastrointestinal bleeding
- visible bleeding
- early and late pregnancy
- drinking more than 250 mL per day: diarrhea, kidney failure
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,022mgP98 national40%
- Magnesiummild; a laxative if you drink itMg²⁺1,191mgP100 national39%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺1,135mgP86 national20%
- Iron (II)Fe²⁺50.0mgP99 national0.7%
- PotassiumK⁺20.3mgP63 national0.2%
- StrontiumSr²⁺15.7mgP95 national0.1%
- AmmoniumNH₄⁺1.5mgP73 national0.0%
- ManganeseMn²⁺0.70mgP72 national0.0%
- BariumBa²⁺0.50mgP83 national0.0%
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,810mgP94 national82%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻3,301mgP99 national18%
- BromideBr⁻22.6mgP94 national0.1%
- HSO4HSO43.1mgP52 national0.0%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂389mgP100 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃153mgP69 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2,631mgP100 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CopperCu0.03mgP93 national
- ArsenicAs0.0030mgP9 national
- 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.
- 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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Ohtsuka Y et al. Transdermal iron absorption from iron-rich hot spring bathing. Japanese J. Balneology, 2003.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.