A sodium chloride hot spring with bicarbonate softness layered in, emerging at 66.3 °C — among Japan's hottest natural sources, cooled before bathing. The salty, heat-retaining chloride character gives the water real body, while 64 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel smoother afterwards.
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
Gut & metabolic
Bicarbonate and sulfate waters, taken as a drink, neutralize stomach acid and help with constipation and gout.
stomach, gout (drinking)
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
Take care
- during diarrhea
- kidney disease
- hypertension
- and other conditions generally accompanied by edema
- iodine-containing springs are contraindicated in hyperthyroidism
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⁺480mgP74 national
- Potassiumminor; matters for drinking limitsK⁺7.3mgP40 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.7mgP11 national
- StrontiumSr²⁺0.18mgP29 national
- LithiumLi⁺0.10mgP27 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻520mgP74 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻360mgP67 national
- Fluoridetrace; drinking limits applyF⁻25.0mgP99 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻20.0mgP67 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1.9mgP11 national
- Thiosulfateoxidized sulfurS₂O₃²⁻1.2mgP74 national
- Bromidetrace, sea-originBr⁻1.1mgP60 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.90mgP69 national
- Iodideyellows in air; helps blood lipidsI⁻0.60mgP73 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂100mgP95 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃64.0mgP41 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.2mgP18 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAsNDmg
- CadmiumCdNDmg
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
On the certificate
- constipation
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.
- Matsumoto S et al. Hot spring bathing and HbA1c in type 2 diabetes. J. JAPMC 73(3):163–170, 2010.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.