A sodium-led chloride spring from the Nakamura no Yu well in Hirayu, with bicarbonate softness layered in. The water rises at 91.0 °C — among Japan's hottest natural sources — and at the spout it runs cloudy with a grey-black cast and a faint sulfur note. The pH is neutral and the mineralization moderate at 1,716 mg/kg, so the water reads as gentle rather than sharp; what stands out is metasilicic acid at 201.9 mg/kg, the kind of silica load that leaves a thin film and places this spring in Japan's beauty-water family.
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⁺373mgP69 national77%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺49.2mgP57 national12%
- Potassiumminor; matters for drinking limitsK⁺59.6mgP87 national7%
- Magnesiummild; a laxative if you drink itMg²⁺9.6mgP61 national4%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- ManganeseMn²⁺0.20mgP46 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻425mgP73 national56%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻503mgP76 national38%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻56.2mgP43 national5%
- FluorideF⁻0.50mgP30 national0.1%
- Hydrogen sulfideHS⁻0.80mgP65 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃202mgP86 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂35.5mgP83 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂24.8mgP54 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.50mgP71 national
- ArsenicAs0.0060mgP18 national
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.