A sodium chloride spring that rises under its own pressure from a 160 m bore in the Ida district of Beppu, arriving at 80.2 °C — among Japan's hottest natural sources. The water is colorless and clear, with only a very faint salty, slightly metallic edge if tasted; it is weakly acidic and hypotonic, carrying a moderate 3.8 g/kg of dissolved solids, and the chloride content is what keeps the warmth in long after the bath. What stands out is the metasilicic acid at 560 mg/kg — an unusually generous figure that leaves the thin silica film associated with Japan's beauty-water family, and skin feels noticeably smoother for it.
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, netto 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⁺1,020mgP86 national86%
- Potassiumminor; matters for drinking limitsK⁺144mgP95 national7%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺41.2mgP54 national4%
- LithiumLi⁺6.6mgP96 national2%
- MagnesiumMg²⁺5.3mgP52 national0.8%
- HydrogenH⁺0.30mgP59 national0.6%
- ManganeseMn²⁺1.1mgP79 national0.1%
- Iron (II)Fe²⁺0.60mgP57 national0.0%
- StrontiumSr²⁺0.40mgP61 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,560mgP86 national84%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻400mgP82 national16%
- FluorideF⁻2.2mgP60 national0.2%
- BromideBr⁻4.6mgP81 national0.1%
- HSO4HSO44.3mgP52 national0.1%
- IodideI⁻0.20mgP51 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃560mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂66.5mgP93 national
- Metaarsenious acidHAsO₂1.7mgP93 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.8mgP20 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.2mgP95 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.
- 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.