Koyasan Onsen, served at Fukuchi-in, is a lightly mineralized simple thermal spring — 171 mg/kg of dissolved solids, weakly alkaline, soft on the skin. The wellhead emerges at only 28.7 °C, so the bathwater is reheated, recirculated through filtration, and chlorinated for sanitary reasons before reaching the tubs.
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
- 急性疾患(特に熱のある場合)
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全
- Renal failure
- Bleeding disorders
- Severe anemia
- その他一般に病勢進行中の疾患
- 妊娠中(とくに初期と末期)
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺22.3mgP41 national62%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺7.9mgP2 national19%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻91.7mgP41 national87%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻5.7mgP16 national7%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3.2mgP5 national5%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.60mgP59 national1%
- FluorideF⁻0.10mgP9 national0.6%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.1mgP17 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂44.1mgP68 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- 神経痛
- Muscle pain
- Joint pain
- Frozen shoulder
- Motor paralysis
- Joint stiffness
- Bruises
- Sprains
- Chronic digestive disorders
- Hemorrhoids
- 冷え症
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.