A hot (48.4 °C) sodium-calcium bicarbonate-chloride spring — the classic 'jūsō-sen' (soda-water) style softened by a salty bicarbonate-leaning chloride sub-type. Weakly alkaline at pH 7.8, gentle on the skin, and sitting squarely in the Japanese 'beauty-water' family. The water carries only a faint yellow tint at the source with virtually no taste or odor. Unusually rich metasilicic acid at 233 mg/kg 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⁺228mgP54 national46%
- Magnesiummild; a laxative if you drink itMg²⁺70.0mgP88 national27%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺90.3mgP69 national21%
- Potassiumminor; matters for drinking limitsK⁺43.4mgP81 national5%
- LithiumLi⁺1.0mgP74 national
- BariumBa²⁺0.20mgP71 national
- AmmoniumNH₄⁺0.05mgP13 national
- Manganesetrace; companion to iron springsMn²⁺0.04mgP22 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.03mgP14 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻911mgP87 national70%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻160mgP50 national21%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻91.2mgP55 national9%
- FluorideF⁻0.40mgP33 national0.1%
- Hydrogen phosphateHPO₄²⁻0.10mgP39 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃233mgP93 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP59 national
- ArsenicAs< —mg
- CadmiumCd< —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.