A sodium-calcium chloride spring emerging hot at 52.4 °C. With roughly 16,270 mg/kg of dissolved solids it is richly mineralized and hypertonic, leaving a salty mineral film on the skin that holds in warmth well after the bath. The water is alkaline on the pH scale, so it feels soft despite the salt.
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⁺3,530mgP95 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,660mgP99 national46%
- PotassiumK⁺24.0mgP68 national0.2%
- StrontiumSr²⁺25.4mgP98 national0.2%
- MagnesiumMg²⁺0.20mgP14 national0.0%
- LithiumLi⁺0.07mgP19 national0.0%
- Iron (II)Fe²⁺0.05mgP17 national0.0%
- ManganeseMn²⁺0.05mgP22 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻9,870mgP96 national100%
- BromideBr⁻38.1mgP96 national0.2%
- CarbonateCO₃²⁻13.1mgP57 national0.2%
- BicarbonateHCO₃⁻14.8mgP10 national0.1%
- IodideI⁻5.5mgP97 national0.0%
- FluorideF⁻0.20mgP17 national0.0%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃28.5mgP11 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂23.7mgP77 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.01mgP52 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.