A blend of six Hirayu wells in the Okuhida hot-spring district — Hosen, Keisen, Wanada-no-yu, Tsuru-no-yu, Horai-no-yu and Choju-no-yu — classified as a simple thermal spring, hypotonic and neutral at pH 6.4. With 875 mg/kg of dissolved solids it is lightly mineralized and gentle on the body, yet it is far from characterless: at the spout the water carries a faint yellow tint and a slight haze, a faintly metallic iron note on the tongue and a whisper of hydrogen sulfide. Metasilicic acid runs to 136 mg/kg, enough to leave the thin silica film that puts this water in Japan's beauty-water family. The certificate was taken at the inn's men's indoor bath inlet rather than at a wellhead, so the reading describes the water as it actually reaches the tub.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺130mgP36 national55%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺57.1mgP60 national27%
- Magnesiummild; a laxative if you drink itMg²⁺14.4mgP67 national11%
- Potassiumminor; matters for drinking limitsK⁺16.7mgP56 national4%
- LithiumLi⁺0.70mgP63 national1.0%
- Iron (II)Fe²⁺1.8mgP71 national0.6%
- AmmoniumNH₄⁺0.70mgP55 national0.4%
- StrontiumSr²⁺0.60mgP73 national0.1%
- ManganeseMn²⁺0.40mgP61 national0.1%
- BariumBa²⁺0.40mgP80 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻372mgP70 national63%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻107mgP43 national31%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻27.2mgP35 national6%
- FluorideF⁻0.50mgP30 national0.3%
- BromideBr⁻0.40mgP41 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃136mgP66 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.5mgP53 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂226mgP89 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.