伊豆山127号泉Izusan Spring No. 127
A hot sodium-calcium chloride spring rising from the wellhead at 71.7 °C — among Japan's hottest natural sources. Weakly alkaline, with 6.4 g/kg of dissolved solids, it leaves a noticeable mineral film on the skin and stays warm long after the bath. Sulfate adds a quiet body to the chloride backbone, giving the water real substance.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,319mgP96 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺886mgP83 national
- Potassiumminor; matters for drinking limitsK⁺13.3mgP51 national
- Magnesiummild; a laxative if you drink itMg²⁺3.1mgP39 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.40mgP48 national
- Manganesetrace; companion to iron springsMn²⁺0.40mgP59 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.07mgP33 national
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,232mgP91 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻777mgP93 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻42.6mgP25 national
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃93.6mgP53 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.4mgP39 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂22.8mgP54 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
Measured at the source
On the certificate
- 一般的適応症(浴用): 神経痛、筋肉痛、関節痛、五十肩、運動麻痺、関節のこわばり、うちみ、くじき、慢性消化器病、痔疾、冷え性、病後回復期、疲労回復、健康増進
- 泉質別適応症: きりきず、末梢循環障害、冷え性、うつ状態、皮膚乾燥症
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.