A hot simple thermal spring blended from two of Yukawa Onsen's wells. Mildly alkaline and soft on the skin, it sits in Japan's beauty-water family, helped along by 58.8 mg/kg of metasilicic acid that leaves a thin silica film for a smoother feel. The water is nearly colorless and transparent, with no taste or smell.
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⁺135mgP36 national62%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺74.2mgP64 national34%
- Magnesiummild; a laxative if you drink itMg²⁺3.8mgP42 national3%
- PotassiumK⁺3.7mgP29 national0.8%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻314mgP73 national58%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻140mgP47 national35%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻44.3mgP27 national6%
- CarbonateCO₃²⁻1.7mgP22 national0.5%
- HydroxideOH⁻0.04mgP33 national
- Bromidetrace, sea-originBr⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃58.8mgP34 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.0mgP29 national
- Metaarsenious acidHAsO₂0.09mgP39 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.
- 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.