A sodium chloride spring classified as 'kyoen' (strong-salt) — chloride accounts for some 90% of the anion balance and total dissolved solids reach 27.6 g/kg, approaching ocean salinity. Salty and faintly bitter on the tongue, it leaves a noticeable mineral film on the skin and a long-lasting warmth after the bath.
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
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺8,144mgP99 national75%
- Magnesiummild; a laxative if you drink itMg²⁺939mgP100 national16%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺645mgP95 national7%
- Potassiumminor; matters for drinking limitsK⁺353mgP99 national2%
- Iron (II)Fe²⁺10.7mgP88 national0.1%
- AmmoniumNH₄⁺1.6mgP76 national0.0%
- ManganeseMn²⁺1.2mgP80 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻15,670mgP99 national90%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,882mgP100 national8%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻363mgP69 national1%
- BromideBr⁻40.6mgP98 national0.1%
- Metaarsenious acidHAsO₂193mgP100 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃187mgP82 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂22.6mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂193mgP88 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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.