A simple thermal spring at Shiratori Onsen Shimo-yu in Ebino, Miyazaki. Total dissolved solids are light at 97 mg/kg, classified as a simple thermal water (hypotonic, weakly acidic, hot). Weakly acidic at pH 4.4 and remarkably hot at 70.9 °C — among Japan's hottest natural sources — it carries a faint hydrogen sulfide note and a modest dose of metasilicic acid at 38 mg/kg.
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
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⁺4.7mgP17 national27%
- AmmoniumNH₄⁺3.1mgP86 national20%
- Magnesiummild; a laxative if you drink itMg²⁺2.1mgP35 national20%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺2.8mgP1 national14%
- Aluminumshows up in acidic volcanic waterAl³⁺0.70mgP76 national10%
- Potassiumminor; matters for drinking limitsK⁺1.6mgP19 national5%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.50mgP52 national2%
- ZincZn²⁺0.20mgP84 national1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻34.2mgP35 national81%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻6.7mgP4 national13%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.8mgP3 national6%
- Thiosulfateoxidized sulfurS₂O₃²⁻0.30mgP58 national1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃38.4mgP20 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 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.
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.