A sodium chloride spring — salty and heat-retaining, drawn hot at 47.3 °C. Weakly alkaline and soft on the skin, it carries a generous 168 mg/kg of metasilicic acid, leaving the kind of thin silica film that makes skin feel noticeably smoother afterwards. Colorless, tasteless and odorless at the source.
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⁺352mgP66 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.4mgP35 national5%
- Potassiumminor; matters for drinking limitsK⁺20.5mgP64 national3%
- AmmoniumNH₄⁺0.20mgP30 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP54 national
- BariumBa²⁺0.10mgP53 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- LithiumLi⁺0.10mgP29 national
- Magnesiummild; a laxative if you drink itMg²⁺0.10mgP11 national
- StrontiumSr²⁺0.10mgP26 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- Manganesetrace; companion to iron springsMn²⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻471mgP74 national76%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻147mgP51 national14%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻83.8mgP52 national10%
- FluorideF⁻0.50mgP32 national0.2%
- BromideBr⁻1.2mgP63 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃168mgP75 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.6mgP62 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- H2SO4FreeH2SO4Free0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂16.9mgP49 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.05mgP49 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.