A sodium-led bicarbonate-chloride spring of the classic jūsō-sen style, softened by a salty edge from the chloride side. Neutral pH and moderate mineralization (1,913 mg/kg dissolved solids) give it a gentle feel with strong heat retention. Served as free-flow from a 48.5 °C source, with no dilution, reheating or disinfection.
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⁺466mgP74 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺64.3mgP63 national13%
- Magnesiummild; a laxative if you drink itMg²⁺14.3mgP66 national5%
- Potassiumminor; matters for drinking limitsK⁺19.8mgP62 national2%
- Iron (II)Fe²⁺0.03mgP15 national0.0%
- Manganesetrace; companion to iron springsMn²⁺0.03mgP19 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,007mgP91 national65%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻301mgP64 national34%
- SulfateSO₄²⁻9.7mgP21 national0.8%
- BromideBr⁻1.0mgP59 national0.1%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.04mgP20 national
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- SS²⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂16.1mgP70 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃13.9mgP2 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂231mgP90 national
- ArsenicAs0.0010mgP6 national
- 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.