A simple thermal spring drawn from Well No. 3 at Appi Onsen Patio. The source emerges at 29.8 °C — a cold-mineral threshold spring that is reheated for bathing. Lightly mineralized (379 mg/kg) and weakly alkaline at pH 7.7, the water is gentle on the skin, but a generous 114 mg/kg of metasilicic acid puts it squarely in the Japanese 'beauty-water' family — a fine silica film leaves the skin noticeably smoother afterwards.
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
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, 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⁺55.7mgP15 national72%
- Magnesiummild; a laxative if you drink itMg²⁺5.4mgP52 national13%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.8mgP22 national12%
- Potassiumminor; matters for drinking limitsK⁺3.7mgP30 national3%
- Iron (II)Fe²⁺0.30mgP43 national0.2%
- ManganeseMn²⁺0.20mgP46 national0.2%
- AluminumAl³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻164mgP54 national83%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻17.4mgP30 national11%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻6.7mgP8 national6%
- FluorideF⁻0.30mgP21 national0.6%
- NitrateNO₃⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃114mgP59 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.0mgP24 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.9mgP34 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—mg
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.