A sodium and calcium chloride spring, salty and heat-retaining. With dissolved solids at roughly 16,270 mg/kg it is richly mineralized and leaves a mineral film on the skin — a hypertonic water with a distinctly salty edge at the source. Alkaline in reaction, it is drawn at a free-flowing borehole wellhead, accompanied by natural gas (mainly methane) rising with the water.
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
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⁺3,533mgP95 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,681mgP99 national46%
- PotassiumK⁺24.8mgP69 national0.2%
- StrontiumSr²⁺25.4mgP98 national0.2%
- AmmoniumNH₄⁺6.0mgP92 national0.1%
- Magnesiummild; a laxative if you drink itMg²⁺21.0mgP70 national
- LithiumLi⁺0.07mgP19 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻9,878mgP97 national100%
- BromideBr⁻36.1mgP96 national0.2%
- HSiO3HSiO₃⁻13.1mgP66 national0.2%
- CarbonateCO₃²⁻14.8mgP58 national0.1%
- IodideI⁻5.5mgP97 national0.0%
- Fluoridetrace; drinking limits applyF⁻0.20mgP17 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.20mgP40 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃28.5mgP11 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂23.7mgP77 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.01mgP52 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.
- 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.