A sodium chloride spring (formerly classed as a 'pure saline' spring), salty and heat-retaining. At pH 8.7 it is alkaline with a slippery, soft feel on the skin, placing it in Japan's beauty-water family. The wellhead runs at 97.4 °C — among the hottest natural sources in the country, cooled before it reaches the baths. A generous 156 mg/kg of metasilicic acid leaves a 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, 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⁺374mgP64 national92%
- Potassiumminor; matters for drinking limitsK⁺28.4mgP72 national4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺6.3mgP19 national2%
- LithiumLi⁺1.7mgP89 national1%
- MagnesiumMg²⁺0.70mgP23 national0.3%
- AluminumAl³⁺0.30mgP70 national0.2%
- AmmoniumNH₄⁺0.20mgP29 national0.1%
- StrontiumSr²⁺0.30mgP50 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻413mgP71 national69%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻97.6mgP54 national12%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻112mgP43 national11%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻34.8mgP84 national7%
- FluorideF⁻1.3mgP54 national0.4%
- BromideBr⁻1.2mgP61 national0.1%
- ThiosulfateS₂O₃²⁻0.50mgP60 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃156mgP72 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.1mgP48 national
- Metaarsenious acidHAsO₂0.30mgP73 national
- ArsenicAs0.23mgP88 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.