Tenshin no Oyu is a gentle simple thermal spring drawn from a hot 60 °C source, neutral in pH and lightly mineralized at just 597 mg/kg of dissolved solids. What sets it apart is a generous 245 mg/kg of metasilicic acid, which leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards, placing it in the Japanese beauty-water family. The water is colorless, clear, tasteless and odorless, sampled at the inlet of the family bath.
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⁺93.1mgP24 national82%
- Potassiumminor; matters for drinking limitsK⁺18.6mgP61 national10%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.0mgP15 national4%
- Magnesiummild; a laxative if you drink itMg²⁺1.8mgP35 national3%
- LithiumLi⁺0.20mgP37 national0.6%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻155mgP50 national56%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻55.1mgP28 national34%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻20.6mgP34 national9%
- FluorideF⁻0.40mgP32 national0.4%
- Bromidetrace, sea-originBr⁻0.10mgP17 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃245mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.3mgP26 national
- Metaarsenious acidHAsO₂0.20mgP65 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP57 national
- ArsenicAs0.11mgP74 national
- CadmiumCd< —mg
- CopperCu< —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.