The house source, Osaka Bay Tower Onsen, is a gentle simple thermal water classified as hypotonic, weakly alkaline and hot. Mineralization is light at 0.42 g/kg, so it reads soft rather than assertive on the skin, and bicarbonate carries almost 88% of the anion balance, which rounds the touch out further. Metasilicic acid at 70.8 mg/kg puts it in Japan's beauty-water family — enough silica to leave the thin film that makes skin feel noticeably smoother after a soak.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺89.5mgP26 national96%
- Magnesiummild; a laxative if you drink itMg²⁺0.60mgP21 national1%
- Potassiumminor; matters for drinking limitsK⁺1.8mgP20 national1%
- AmmoniumNH₄⁺0.50mgP49 national0.7%
- CalciumCa²⁺0.40mgP1 national0.5%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- LithiumLi⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- StrontiumSr²⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻236mgP60 national88%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻15.7mgP16 national10%
- Hydrogen phosphateHPO₄²⁻1.9mgP89 national0.9%
- SulfateSO₄²⁻1.7mgP9 national0.8%
- Hydrogen sulfideHS⁻0.80mgP66 national0.6%
- FluorideF⁻0.10mgP9 national0.1%
- BromideBr⁻0.20mgP30 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃70.8mgP44 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.90mgP12 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.0mgP33 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0080mgP19 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —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.