長生源泉Chosei Spring
A hot 78.6 °C sodium bicarbonate-chloride spring among Japan's hotter natural sources, cooled before bathing. Weakly alkaline and soft on the skin, it carries a faint salty edge and notable body from 5,063 mg/kg of dissolved solids, with bicarbonate softness layered against the salt. A generous 110 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺1,486mgP89 national93%
- Potassiumminor; matters for drinking limitsK⁺73.0mgP89 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺32.4mgP48 national2%
- Magnesiummild; a laxative if you drink itMg²⁺9.5mgP60 national1%
- CopperCu²⁺0.0028mgP46 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,018mgP96 national48%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,071mgP81 national44%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻149mgP99 national7%
- SulfateSO₄²⁻19.0mgP30 national0.6%
- FluorideF⁻6.9mgP84 national0.5%
- HSiO3HSiO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃110mgP59 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂71.7mgP93 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂27.3mgP59 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
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.