A sodium chloride spring from the Ichinoide district of Beppu: salty, heat-retaining water. Alkaline with the slippery feel of Japan's beauty-water family, it is moderately mineralized with noticeable body and leaves a faint salty tang on the lips. Among Japan's hottest natural sources, it carries metasilicic acid at 254 mg/kg, leaving a thin silica film for noticeably smoother skin.
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⁺769mgP83 national84%
- Potassiumminor; matters for drinking limitsK⁺111mgP93 national7%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺54.3mgP59 national7%
- LithiumLi⁺6.1mgP96 national2%
- MagnesiumMg²⁺1.5mgP30 national0.3%
- AmmoniumNH₄⁺0.80mgP60 national0.1%
- StrontiumSr²⁺0.40mgP61 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,270mgP83 national86%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻199mgP65 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻30.0mgP83 national2%
- BicarbonateHCO₃⁻20.1mgP13 national0.8%
- FluorideF⁻1.5mgP54 national0.2%
- BromideBr⁻3.8mgP79 national0.1%
- IodideI⁻0.20mgP51 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃254mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂45.0mgP86 national
- Metaarsenious acidHAsO₂1.4mgP91 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.94mgP93 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.