A sodium-calcium chloride spring — salty and strongly heat-retaining. It is a blend of seven wells gathered in a holding tank, fed by an exceptionally hot source at 89.5 °C, among the hottest natural waters in Japan. Richly mineralized at 16.33 g/kg of dissolved solids, it leaves a mineral film on the skin, while a mildly alkaline pH keeps the feel soft.
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⁺3,710mgP95 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,168mgP98 national
- Potassiumminor; matters for drinking limitsK⁺323mgP99 national
- StrontiumSr²⁺15.8mgP96 national
- Magnesiummild; a laxative if you drink itMg²⁺5.9mgP54 national
- BariumBa²⁺2.3mgP95 national
- LithiumLi⁺0.70mgP64 national
- Manganesetrace; companion to iron springsMn²⁺0.20mgP45 national
- AmmoniumNH₄⁺0.20mgP30 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻9,721mgP96 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻141mgP60 national
- Bromidetrace, sea-originBr⁻29.0mgP94 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻19.9mgP12 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.1mgP42 national
- Fluoridetrace; drinking limits applyF⁻1.5mgP54 national
- Iodideyellows in air; helps blood lipidsI⁻0.10mgP37 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃182mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.5mgP44 national
- ArsenicAs0.0080mgP19 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.