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
From a certificate analyzed in 2015.
- Active stage of any illness (especially with fever), active tuberculosis, advanced cancer, severe anemia or other marked physical weakness, severe heart or lung disease causing shortness of breath on slight exertion, severe kidney disease with edema, gastrointestinal bleeding, visible bleeding, acute exacerbation of a chronic illness
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⁺194mgP51 national68%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺70.7mgP64 national29%
- Magnesiummild; a laxative if you drink itMg²⁺5.3mgP52 national4%
- PotassiumK⁺1.6mgP18 national0.3%
- StrontiumSr²⁺0.30mgP52 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⁻309mgP66 national70%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻146mgP52 national25%
- Fluoridetrace; drinking limits applyF⁻5.6mgP79 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
From a certificate analyzed in 2015.
- Chronic muscle or joint pain or stiffness (the chronic phase of rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like), muscle stiffness associated with motor paralysis, cold sensitivity, peripheral circulation disorders, reduced gastrointestinal function (heaviness, bloating), mild hypertension, impaired glucose tolerance (diabetes), mild hypercholesterolemia, mild asthma or emphysema, hemorrhoid pain, autonomic nervous system instability, stress-related symptoms (sleep disorders, depressed mood), convalescence, fatigue recovery, general health improvement
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.