A sodium-chloride spring rising along the Ibusuki coast — salty, heat-retaining water with the body of a richly mineralized source (12,610 mg/kg total dissolved solids). At 79.3 °C it ranks among Japan's hottest natural sources, cooled before bathing. An unusually generous 205 mg/kg of metasilicic acid leaves a thin silica film that smooths the skin afterwards.
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⁺3,800mgP96 national78%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺565mgP93 national13%
- Magnesiummild; a laxative if you drink itMg²⁺150mgP95 national6%
- Potassiumminor; matters for drinking limitsK⁺244mgP98 national3%
- LithiumLi⁺2.5mgP93 national0.2%
- AmmoniumNH₄⁺1.8mgP79 national0.1%
- StrontiumSr²⁺4.0mgP88 national0.0%
- ManganeseMn²⁺1.5mgP86 national0.0%
- Iron (II)Fe²⁺0.30mgP43 national0.0%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻6,957mgP95 national94%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻459mgP85 national5%
- BicarbonateHCO₃⁻109mgP44 national0.9%
- CarbonateCO₃²⁻9.6mgP50 national0.1%
- BromideBr⁻23.6mgP94 national0.1%
- FluorideF⁻0.80mgP44 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃205mgP86 national
- Metaboric acidHBO₂31.1mgP80 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxideCO₂36.6mgP64 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.