A hot sodium chloride spring drawn from the Hotel Fugetsu well in Beppu, rising at 84.8 °C — among Japan's hottest natural sources. The water is weakly acidic and moderately mineralized at 3,528 mg/kg of dissolved solids, with a faintly salty taste and a light metallic note at the sampling point. Chloride water of this kind holds heat well after the bath. It also carries a generous 387 mg/kg of metasilicic acid, the silica that leaves a thin film on the skin and puts this source in Japan's beauty-water family.
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⁺965mgP84 national84%
- Potassiumminor; matters for drinking limitsK⁺147mgP95 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.0mgP52 national4%
- LithiumLi⁺10.5mgP98 national3%
- MagnesiumMg²⁺5.5mgP52 national0.9%
- HydrogenH⁺0.10mgP53 national0.2%
- FeTotalFe1.6mgP77 national0.1%
- ManganeseMn²⁺1.3mgP82 national0.1%
- AmmoniumNH₄⁺0.90mgP63 national0.1%
- StrontiumSr²⁺0.20mgP42 national0.0%
- BariumBa²⁺0.10mgP51 national0.0%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,446mgP85 national81%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻454mgP84 national19%
- FluorideF⁻1.8mgP58 national0.2%
- BromideBr⁻6.0mgP85 national0.2%
- HSO4HSO41.8mgP50 national0.0%
- IodideI⁻0.30mgP61 national0.0%
- H2PO4H₂PO₄⁻0.20mgP56 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃387mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂56.6mgP91 national
- Metaarsenious acidHAsO₂1.8mgP94 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂41.7mgP65 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.