A cold-mineral spring (under 25 °C) drawn from the hills of Uonuma, Niigata, reheated for bathing. A sodium-led chloride water layered with a softening bicarbonate sub-type — weakly alkaline, with a faint salt tang on the lips and a trace of rotten-egg note from H₂S. The metasilicic acid content (82.7 mg/kg) leaves the skin noticeably smooth.
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⁺431mgP72 national98%
- PotassiumK⁺7.0mgP39 national0.9%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺0.79mgP2 national
- Magnesiummild; a laxative if you drink itMg²⁺0.70mgP23 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻484mgP74 national72%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻427mgP73 national26%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃82.7mgP48 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.