A sodium chloride hot spring, steam-conditioned by blending several Miyagino wellheads with natural spring water. The source emerges at 76.5 °C — among the hotter natural sources — and its moderate salinity gives the salty, heat-retaining body typical of chloride water. A generous 121 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel smoother. The water is colorless, clear, tasteless and odorless, and is diluted with water to bring it to a comfortable bathing temperature.
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⁺230mgP57 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺36.1mgP49 national
- Potassiumminor; matters for drinking limitsK⁺21.7mgP66 national
- Magnesiummild; a laxative if you drink itMg²⁺7.3mgP58 national
- Manganesetrace; companion to iron springsMn²⁺1.2mgP81 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.31mgP44 national
- LithiumLi⁺0.26mgP38 national
- StrontiumSr²⁺0.21mgP42 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.06mgP32 national
- ZincZn²⁺0.03mgP56 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻364mgP71 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻125mgP47 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻93.0mgP54 national
- Bromidetrace, sea-originBr⁻0.71mgP52 national
- HSiO3HSiO₃⁻0.61mgP51 national
- BO2BO₂⁻0.30mgP43 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.27mgP12 national
- Fluoridetrace; drinking limits applyF⁻0.26mgP16 national
- NitrateNO₃⁻0.11mgP26 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃121mgP62 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂19.5mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.3mgP35 national
- ArsenicAs0.26mgP84 national
- CadmiumCdNDmg
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.