A sodium chloride spring, printed on the certificate under its older name jun-shokuen-sen ("pure saline spring"), and drawn from one of Japan's hottest natural sources. Alkaline chemistry gives it the slippery, soft-on-the-skin feel of Japan's beauty-water family, while the chloride load helps the water hold heat long after you get out. A generous 194 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother.
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, netto 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⁺522mgP78 national94%
- Potassiumminor; matters for drinking limitsK⁺35.0mgP79 national4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺5.7mgP18 national1%
- LithiumLi⁺1.4mgP87 national0.8%
- AluminumAl³⁺0.30mgP66 national0.1%
- StrontiumSr²⁺0.10mgP24 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻601mgP77 national66%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻98.4mgP98 national13%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻144mgP59 national12%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻89.1mgP41 national6%
- BO2BO₂⁻27.0mgP98 national2%
- FluorideF⁻2.9mgP66 national0.6%
- BromideBr⁻1.6mgP69 national0.1%
- HydroxideOH⁻0.30mgP74 national0.1%
- AsO2AsO₂⁻0.70mgP98 national0.0%
- Hydrogen sulfideHS⁻0.10mgP26 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃194mgP84 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.52mgP91 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.