おかたの湯Okata-no-yu Hot Spring
A hot simple thermal spring (71.1 °C, neutral pH) — light in mineralization at 673 mg/kg dissolved solids, yet rich in metasilicic acid (195.7 mg/kg). Colorless and clear at the wellhead; by 24 hours after sampling the water has taken on a faint yellow-brown tint and a mild mineral note. The silica leaves the kind of thin film that makes skin feel noticeably smoother — the source sits squarely in the Japanese 'beauty-water' family.
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
Take care
From a certificate analyzed in 2022.
- 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
- 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⁺93.5mgP26 national66%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺30.8mgP45 national25%
- Potassiumminor; matters for drinking limitsK⁺9.9mgP45 national4%
- ManganeseMn²⁺0.20mgP47 national0.2%
- StrontiumSr²⁺0.20mgP41 national0.2%
- LithiumLi⁺0.01mgP11 national0.2%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Magnesiummild; a laxative if you drink itMg²⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻270mgP63 national72%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻39.6mgP38 national13%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻28.9mgP20 national13%
- FluorideF⁻0.50mgP30 national0.5%
- ThiosulfateS₂O₃²⁻1.2mgP74 national0.3%
- Hydrogen sulfideHS⁻0.70mgP62 national0.2%
- BromideBr⁻0.10mgP14 national0.0%
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃196mgP84 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.1mgP12 national
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2022.
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Cold sensitivity
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders, depressed mood)
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.