A sodium-calcium chloride spring and one of Japan's hottest natural sources at 79.6 °C. Richly mineralized at 10.89 g/kg of dissolved solids, it leaves a salty, heat-retaining mineral film on the skin. A modest 92 mg/kg of metasilicic acid adds a touch of smoothness, and the water sits at a neutral pH.
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⁺2,397mgP92 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,475mgP97 national
- Potassiumminor; matters for drinking limitsK⁺153mgP95 national
- Magnesiummild; a laxative if you drink itMg²⁺39.0mgP82 national
- StrontiumSr²⁺9.6mgP94 national
- Manganesetrace; companion to iron springsMn²⁺2.1mgP91 national
- AmmoniumNH₄⁺1.6mgP76 national
- LithiumLi⁺0.80mgP70 national
- BariumBa²⁺0.30mgP77 national
- Iron (III)Fe³⁺0.20mgP66 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5,803mgP94 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻830mgP93 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻59.7mgP32 national
- Bromidetrace, sea-originBr⁻15.3mgP91 national
- Fluoridetrace; drinking limits applyF⁻1.1mgP49 national
- Iodideyellows in air; helps blood lipidsI⁻0.40mgP66 national
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃91.9mgP52 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.6mgP48 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂14.9mgP47 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.