A sodium chloride spring — salty and heat-retaining, though well below ocean salinity. Alkaline at pH 8.4, it has the soft, slippery feel of Japan's beauty-water family. The water carries a faint yellow-brown tint and a slight metallic iron note at the source, and with dissolved solids around 3,309 mg/kg it sits in moderately mineralized territory with noticeable body.
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⁺1,100mgP86 national93%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,510mgP84 national83%
- Fluoridetrace; drinking limits applyF⁻9.5mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂71.5mgP93 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃25.0mgP9 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.
- 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.