A blend of five Nakao Onsen wells (No. 1, 4, 5, 6 and 8), this is a sodium chloride-bicarbonate spring that comes up at a remarkable 81.0 °C — among Japan's hottest natural sources. It is neutral and hypotonic, colorless and clear, yet it carries a faint sulfur note and a scattering of yunohana mineral flecks drifting in the water. What really stands out is metasilicic acid at 441 mg/kg: that much silica puts the water squarely in Japan's beauty-water family, leaving a thin film that makes skin feel noticeably smoother. The sample was drawn from the distribution tank where the five wells arrive blended, rather than at any single wellhead.
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⁺261mgP59 national82%
- Potassiumminor; matters for drinking limitsK⁺30.9mgP76 national6%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺13.4mgP30 national5%
- LithiumLi⁺3.5mgP93 national4%
- Magnesiummild; a laxative if you drink itMg²⁺2.6mgP38 national2%
- Aluminumshows up in acidic volcanic waterAl³⁺1.8mgP80 national1%
- Iron (II)Fe²⁺2.0mgP74 national0.5%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻235mgP59 national45%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻385mgP71 national43%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻50.0mgP41 national7%
- Fluoridetrace; drinking limits applyF⁻11.4mgP93 national4%
- Hydrogen sulfideHS⁻1.4mgP72 national0.3%
- NitrateNO₃⁻0.10mgP25 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃441mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂31.0mgP80 national
- Metaarsenious acidHAsO₂1.3mgP89 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂141mgP83 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.60mgP71 national
- ArsenicAs0.93mgP93 national
- CadmiumCd0.04mgP90 national
- LeadPb0.02mgP77 national
- ChromiumCr0.0050mgP67 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0011mgP81 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.