A simple thermal spring drawn from Miharayama Onsen's No. 5 well at a remarkable 84 °C — one of the hotter natural sources on Izu Oshima, cooled before bathing. Neutral pH, hypotonic, and lightly mineralized (0.85 g/kg dissolved solids), so the water reads as quietly gentle on the skin rather than mineral-heavy. What sets it apart is a generous 333.8 mg/kg of metasilicic acid: enough to leave the thin silica film that gives the classic Japanese "beauty-water" feel, smoothing the skin after a soak. Colourless and clear at the wellhead, with no odour and a faint bitter note on the tongue.
Good for
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
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺56.4mgP59 national44%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺43.9mgP11 national29%
- Magnesiummild; a laxative if you drink itMg²⁺15.6mgP67 national20%
- Potassiumminor; matters for drinking limitsK⁺17.2mgP57 national7%
- Manganesetrace; companion to iron springsMn²⁺0.30mgP53 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻268mgP62 national70%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻37.4mgP25 national17%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻28.3mgP34 national9%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Fluoridetrace; drinking limits applyF⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃334mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂38.5mgP84 national
- H2SO4FreeH2SO4Free< —mg
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂56.5mgP71 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0060mgP17 national
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.