A hot (43.9 °C) sodium bicarbonate-chloride spring — the soft, skin-smoothing 'jūsō-sen' style with a salty chloride finish layered in. Neutral pH and a moderate 2,356 mg/kg of dissolved solids give it body without heaviness, and 65 mg/kg of metasilicic acid sits firmly in beauty-water territory, leaving skin noticeably smoother after a soak.
Good for
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
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
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
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⁺581mgP77 national93%
- Potassiumminor; matters for drinking limitsK⁺27.3mgP72 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.0mgP32 national3%
- Magnesiummild; a laxative if you drink itMg²⁺4.6mgP49 national1%
- LithiumLi⁺1.1mgP76 national0.6%
- AmmoniumNH₄⁺1.3mgP68 national0.3%
- Iron (II)Fe²⁺0.04mgP16 national0.0%
- StrontiumSr²⁺0.01mgP9 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,398mgP93 national80%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻189mgP54 national19%
- Fluoridetrace; drinking limits applyF⁻6.4mgP85 national1%
- BromideBr⁻0.60mgP50 national0.0%
- Hydrogen sulfideHS⁻0.30mgP46 national0.0%
- SulfateSO₄²⁻0.50mgP5 national0.0%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂66.0mgP92 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃65.5mgP43 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂93.1mgP79 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 national
- ArsenicAs< —mg
- CopperCu< —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.