A sodium-calcium chloride strong-salt spring, densely mineralized to a degree that exceeds ocean salinity and leaves a thick salt coating on the skin. The water shows a faint yellow tint with slight turbidity at the source, a strongly salty and bitter character, and a faint sulfurous note. Weakly alkaline and hypertonic, it is the kind of brine that holds heat well and keeps the body warm long after bathing.
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⁺5,590mgP97 national62%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,751mgP99 national35%
- Magnesiummild; a laxative if you drink itMg²⁺132mgP93 national3%
- PotassiumK⁺94.0mgP92 national0.6%
- ManganeseMn²⁺5.3mgP97 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻13,920mgP98 national100%
- BicarbonateHCO₃⁻38.7mgP23 national0.2%
- BromideBr⁻17.1mgP92 national0.1%
- Hydrogen sulfideHS⁻0.30mgP47 national0.0%
- IodideI⁻0.30mgP62 national0.0%
- SulfateSO₄²⁻0.30mgP4 national0.0%
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂47.4mgP87 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃20.9mgP6 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂30.4mgP60 national
- ArsenicAs< —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.