弥次ヶ湯12号Yajigayu Spring No. 12
Yajigayu Spring No. 12 in Ibusuki is a sodium chloride spring — salty and heat-retaining, with neutral pH and 3,006 mg/kg of dissolved solids giving it noticeable body. A generous 175 mg/kg of metasilicic acid leaves a thin silica film that makes the skin feel smoother afterward.
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⁺822mgP82 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺141mgP79 national15%
- Potassiumminor; matters for drinking limitsK⁺90.2mgP91 national5%
- Magnesiummild; a laxative if you drink itMg²⁺9.9mgP61 national2%
- Iron (II)Fe²⁺2.3mgP74 national0.2%
- LithiumLi⁺4.7mgP95 national0.2%
- StrontiumSr²⁺0.10mgP26 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,545mgP84 national92%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻104mgP55 national5%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻88.0mgP40 national3%
- BromideBr⁻6.6mgP85 national0.2%
- Fluoridetrace; drinking limits applyF⁻0.10mgP9 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃175mgP78 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.9mgP72 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP58 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.