A simple thermal spring drawn from the "Yoshimatsu Kawahigashi No. 5" source in Tsurumaru, Yusui. The water emerges hot at 62.5 °C and is mildly alkaline, soft on the skin, with a faint pale-yellow tint. It is unusually rich in metasilicic acid (170.5 mg/kg), which leaves the kind of silica film that makes skin feel noticeably smoother and places it squarely in Japan's beauty-water family.
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⁺67.4mgP21 national61%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.7mgP29 national13%
- Magnesiummild; a laxative if you drink itMg²⁺6.1mgP55 national10%
- AmmoniumNH₄⁺6.6mgP93 national8%
- Potassiumminor; matters for drinking limitsK⁺13.6mgP52 national7%
- Iron (II)Fe²⁺0.20mgP39 national0.2%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻269mgP62 national96%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4.2mgP6 national3%
- Hydrogen phosphateHPO₄²⁻1.3mgP84 national0.7%
- SulfateSO₄²⁻0.70mgP6 national0.2%
- NitrateNO₃⁻0.0000mg0.2%
- FluorideF⁻—mg0.2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃171mgP77 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.80mgP11 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.2mgP56 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —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.