A sodium-led bicarbonate spring — the classic jūsō-sen style, soft and skin-smoothing on contact. Neutral pH with moderate mineralization (about 1,600 mg/kg dissolved solids) gives the water noticeable body without harshness. A generous 122 mg/kg of metasilicic acid leaves the thin silica film that puts this firmly in Japan's beauty-water family.
Good for
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
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
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⁺375mgP67 national83%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺31.9mgP47 national8%
- Magnesiummild; a laxative if you drink itMg²⁺18.1mgP69 national8%
- PotassiumK⁺7.2mgP39 national0.9%
- LithiumLi⁺0.40mgP50 national0.3%
- Iron (II)Fe²⁺0.60mgP56 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- FeTotalFe0.04mgP23 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻797mgP86 national69%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻140mgP60 national15%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻102mgP40 national15%
- FluorideF⁻0.40mgP31 national0.1%
- BromideBr⁻0.30mgP36 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃122mgP62 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.2mgP44 national
- H2SO4FreeH2SO4FreeNDmg
- Phosphoric acidH₃PO₄NDmg
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂73.0mgP75 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.