A hot (50.2 °C) simple thermal spring in Ebino, Miyazaki, served from a mixing tank on the city-run Shiratori upper-bath line. Weakly acidic in cert terms — pH 3.8 at the wellhead, falling to pH 3.43 in the lab — the water shows a faint milky turbidity, a slight metallic-and-sour edge on the tongue, and a soft hydrogen-sulfide note. Total dissolved solids are light at 170 mg/kg, but a modest 64 mg/kg of metasilicic acid leaves the skin feeling noticeably smoother on emerge.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.3mgP22 national22%
- AmmoniumNH₄⁺4.8mgP92 national16%
- Magnesiummild; a laxative if you drink itMg²⁺2.9mgP38 national15%
- Aluminumshows up in acidic volcanic waterAl³⁺1.8mgP80 national12%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.20mgP59 national12%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺4.8mgP80 national10%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺3.4mgP1 national9%
- Potassiumminor; matters for drinking limitsK⁺0.10mgP1 national3%
- ZincZn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻77.0mgP50 national97%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.8mgP3 national3%
- HSO4HSO40.40mgP42 national
- Thiosulfateoxidized sulfurS₂O₃²⁻0.10mgP45 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃63.7mgP39 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.10mgP5 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂48.5mgP68 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national
- ArsenicAs< —mg
- 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.