A sodium chloride hot spring at 45.7 °C, isotonic and near-neutral (pH 6.0 at the wellhead). Moderately mineralized at 9.87 g/kg, with noticeable body — salty, heat-retaining chloride water. Metasilicic acid is generous at 152 mg/kg, leaving the kind of thin silica film that makes skin feel smoother afterwards, in the Japanese "beauty-water" family. The lab sample develops a faint iron note on standing, hinting at the trace iron the cert reports.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺2,917mgP93 national74%
- Magnesiummild; a laxative if you drink itMg²⁺267mgP97 national15%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺338mgP90 national10%
- Potassiumminor; matters for drinking limitsK⁺136mgP94 national2%
- ManganeseMn²⁺2.8mgP93 national0.1%
- AmmoniumNH₄⁺0.40mgP43 national0.0%
- AluminumAl³⁺0.10mgP54 national0.0%
- FeTotalFe0.07mgP33 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,196mgP93 national90%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻516mgP88 national7%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻367mgP70 national4%
- BromideBr⁻15.2mgP91 national0.1%
- IodideI⁻0.70mgP75 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃152mgP69 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂58.7mgP91 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂239mgP90 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —mg
- CopperCuNDmg
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.