A hot (75.5 °C) sodium chloride-bicarbonate blend, drawing on two wells (Miyuki Motoyu and Miyuki No.2) into one tapped source. The water is hypertonic at 11.12 g/kg dissolved solids and weakly alkaline, carrying a clear salty edge and a faint rotten-egg whisper from trace hydrogen sulfide. With 89.4 mg/kg of metasilicic acid it also runs in the soft, slightly slippery 'beauty-water' direction.
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⁺3,178mgP94 national83%
- Magnesiummild; a laxative if you drink itMg²⁺199mgP96 national10%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺155mgP80 national5%
- Potassiumminor; matters for drinking limitsK⁺24.8mgP69 national2%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,515mgP92 national73%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,318mgP97 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻466mgP85 national6%
- FluorideF⁻0.40mgP31 national0.0%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃89.4mgP51 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂51.2mgP89 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂12.7mgP44 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —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.