A simple thermal spring at Hirano on the shore of Lake Yamanaka, pumped to the surface and sitting on the alkaline side at roughly pH 9, which gives it the slippery feel of Japan's beauty-water family. Lightly mineralized at 176 mg/kg, it is colorless, clear, tasteless, and odorless — gentle and unassuming on the skin. The source emerges at 29.2 °C, a cold-mineral-range temperature, so it is reheated for bathing.
Good for
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
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⁺46.9mgP12 national78%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.3mgP9 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.4mgP2 national1%
- Fluoridetrace; drinking limits applyF⁻0.04mgP4 national1%
- HSiO3HSiO₃⁻42.9mgP88 national
- HydroxideOH⁻3.3mgP97 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃36.6mgP18 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.
- 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.