A weakly alkaline simple thermal spring rising at 36.7 °C, colourless and odourless with no notable taste — the kind of unfussy, low-irritation water that suits long soaks. Total dissolved solids sit at a light 890 mg/kg, with sodium and calcium leading the cations and chloride balanced against bicarbonate among the anions, so nothing dominates the chemistry. The pH of 8.5 lands in 'beauty-water' territory: mildly alkaline, soft on the skin, with a faintly slippery feel. The source feeds the Ishiyu communal bath in Yunohana Onsen — a self-flowing well, served plainly as a gentle, everyday hot spring rather than a high-mineral cure-water.
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
Take care
- 病気の活動期(特に熱のあるとき)、活動性の結核、進行した癌または高度の貧血など身体衰弱の著しい場合、少しの運動で息切れする重い心臓または肺の病気、むくみのあるような重い腎臓の病気、消化管出血、目に見える出血があるとき、慢性の病気の急性増悪期
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⁺194mgP50 national68%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺70.7mgP64 national29%
- Magnesiummild; a laxative if you drink itMg²⁺5.3mgP51 national4%
- PotassiumK⁺1.6mgP19 national0.3%
- StrontiumSr²⁺0.30mgP55 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- LithiumLi⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- AmmoniumNH₄⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻309mgP64 national70%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻146mgP51 national25%
- Fluoridetrace; drinking limits applyF⁻5.6mgP80 national2%
- Bromidetrace, sea-originBr⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSO4HSO4< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃80.4mgP47 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.3mgP59 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.17mgP77 national
- Radonfaint radon — studied for joint pain (hormesis)RnNDmg
Measured at the source
On the certificate
- 筋肉若しくは関節の慢性的な痛み又はこわばり(関節リウマチ、変形性関節症、腰痛症、神経痛、五十肩、打撲、捻挫などの慢性期)、運動麻痺における筋肉のこわばり、冷え性、末梢循環障害、胃腸機能の低下(胃がもたれる、腸にガスがたまるなど)、軽症高血圧、耐糖能異常(糖尿病)、軽い高コレステロール血症、軽い喘息又は肺気腫、痔の痛み、自律神経不安定症、ストレスによる諸症状(睡眠障害、うつ状態など)、病後回復期、疲労回復、健康増進
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.