A gentle simple thermal water, weakly alkaline and soft on the skin, nearly colorless, transparent, tasteless and odorless at the source. It carries a generous 56.7 mg/kg of metasilicic acid, leaving the kind of thin silica film that makes skin feel smoother and placing it in Japan's beauty-water family. Mineralization is light, so the water sits easy on the body.
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⁺136mgP37 national65%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺62.3mgP62 national34%
- PotassiumK⁺3.5mgP29 national1.0%
- 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
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻316mgP75 national73%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻72.1mgP33 national22%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻26.4mgP16 national5%
- BromideBr⁻0.10mgP15 national0.0%
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃56.7mgP34 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.5mgP23 national
- Metaarsenious acidHAsO₂0.07mgP39 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.6mgP23 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.