Matsuki no Yu rises in the Kitahama district of Beppu as a sodium chloride spring — jun-shokuen-sen under the older naming. Hypotonic and mildly alkaline, it runs colorless and clear at the wellhead with only the faintest salty tang on the lips, and the chloride load gives it the heat-retaining quality the type is known for. A generous 180.3 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably smoother afterwards.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺848mgP83 national78%
- Magnesiummild; a laxative if you drink itMg²⁺48.0mgP85 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺78.2mgP66 national8%
- Potassiumminor; matters for drinking limitsK⁺104mgP92 national6%
- ManganeseMn²⁺2.7mgP93 national0.2%
- Iron (III)Fe³⁺0.60mgP80 national0.1%
- AluminumAl³⁺0.10mgP54 national0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,409mgP83 national79%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻520mgP77 national17%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻95.7mgP54 national4%
- CarbonateCO₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃180mgP80 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.5mgP39 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂23.0mgP54 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.02mgP35 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.