An alkaline simple thermal spring, weakly mineralized and gentle on the body. At pH 8.7 from the source it sits in the alkaline range, giving the slippery feel of Japan's beauty-water family. The water is colorless and clear with no taste, carrying only a faint rotten-egg note of hydrogen sulfide.
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⁺146mgP39 national70%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺52.2mgP58 national29%
- PotassiumK⁺1.9mgP22 national0.6%
- StrontiumSr²⁺0.30mgP55 national0.1%
- LithiumLi⁺0.20mgP40 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻292mgP73 national65%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻95.4mgP39 national29%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻19.9mgP12 national3%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻8.1mgP45 national3%
- FluorideF⁻1.1mgP49 national0.6%
- HydroxideOH⁻0.30mgP74 national0.1%
- Bromidetrace, sea-originBr⁻0.30mgP36 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃47.0mgP26 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.7mgP44 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂433mgP95 national
- ArsenicAs0.19mgP78 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.