A sodium-calcium chloride spring — heat-retaining salt water served at a brisk 42 degC. Weakly alkaline and easy on the skin, it carries about 1,735 mg/kg of dissolved solids, with chloride at 822 mg/kg providing the noticeable salty body and long-lasting warmth typical of the type.
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, nettō 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⁺470mgP75 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺121mgP77 national
- Potassiumminor; matters for drinking limitsK⁺16.4mgP56 national
- StrontiumSr²⁺2.4mgP86 national
- Magnesiummild; a laxative if you drink itMg²⁺1.0mgP26 national
- LithiumLi⁺0.20mgP40 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻823mgP79 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻192mgP64 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻57.3mgP31 national
- Bromidetrace, sea-originBr⁻2.7mgP75 national
- Iodideyellows in air; helps blood lipidsI⁻1.0mgP79 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.0mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.8mgP55 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.2mgP27 national
Measured at the source
On the certificate
- 萎縮性胃炎
- 便秘
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.