A leg suddenly swells, feels tight, and is tender to the touch — seek medical care promptly; if shortness of breath or chest pain develops suddenly, call 120 immediately.

If one leg suddenly swells, becomes noticeably thicker than the other, and is painful to press, get a leg vein ultrasound done as soon as possible. Should shortness of breath, chest pain when taking a deep breath, a rapid heartbeat, or dizziness appear, a blood clot may have traveled to the lungs — call 120 right away. Among people diagnosed with pulmonary embolism, 17.4% die within three months. Sitting still for more than four hours roughly doubles the risk, though the actual chance for any one person remains very low.

An ultrasound of the leg veins (lower extremity venous ultrasound) costs around 100–200 yuan.

The International Cooperative Registry on Pulmonary Embolism (ICOPER) gathered data from 2,454 patients with confirmed acute pulmonary embolism treated at 52 hospitals across seven countries. Pulmonary embolism occurs when a blood clot blocks a pulmonary artery. Within three months, the overall mortality rate for these patients — regardless of cause — was 17.4% (426 out of 2,454). The WHO WRIGHT project found that travel lasting four or more continuous hours “roughly doubles” the risk of blood clots forming in veins and traveling to the lungs (venous thromboembolism). Even so, the actual probability for any individual remains low; prolonged inactivity exceeding four hours translates to roughly one in 6,000 people facing this risk.

Swelling in both legs is usually linked to issues involving the heart, liver, kidneys, or venous function. Sudden swelling of just one leg, noticeably larger than the other, is a classic sign of deep vein thrombosis. High-risk situations include long car or plane trips, bed rest after surgery or a fracture, wearing a cast, pregnancy and the postpartum period, cancer, and oral contraceptive use. When a clot breaks loose and blocks a pulmonary artery, pulmonary embolism results — marked by sudden shortness of breath, worsening chest pain with deep breaths, rapid heartbeat, dizziness, and sometimes coughing up blood. In such cases, call 120 immediately. The 17.4% figure reflects mortality among confirmed pulmonary embolism cases, not the overall risk associated with leg swelling alone; don’t let this number cause undue alarm.

Goldhaber SZ, Visani L, De Rosa M (1999). Acute pulmonary embolism: clinical outcomes in the International Cooperative Pulmonary Embolism Registry (ICOPER). Lancet, 353(9162), 1386-1389. https://doi.org/10.1016/S0140-6736(98)07534-5 ; WHO (2007). Study results released on travel and blood clots. https://www.who.int/news/item/29-06-2007-study-results-released-on-travel-and-blood-clots ; NHS. DVT (deep vein thrombosis)：「If you're travelling for 4 hours or more by plane, train or car」「wearing loose clothing」「drinking plenty of water」「avoiding alcohol」「walking around when possible」. https://www.nhs.uk/conditions/deep-vein-thrombosis-dvt/ ; Clarke MJ, Broderick C, Hopewell S, Juszczak E, Eisinga A (2021). Compression stockings for preventing deep vein thrombosis in airline passengers. Cochrane Database of Systematic Reviews, 4, CD004002. https://doi.org/10.1002/14651858.CD004002.pub4