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

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

The International Cooperative Pulmonary Embolism Registry (ICOPER) enrolled 2,454 patients with confirmed acute pulmonary embolism at 52 hospitals in 7 countries. Pulmonary embolism is a blood clot blocking a pulmonary artery. All-cause mortality within 3 months was 17.4% (426/2,454). The WHO WRIGHT project found that uninterrupted travel lasting 4 hours or more roughly doubles the risk of venous thromboembolism: clots forming in veins and travelling to the lungs. The absolute risk remains low, about 1 in 6,000 after sitting still for more than 4 hours. The UK NHS recommends loose clothing, plenty of water, no alcohol and getting up to walk when possible on plane, train or car journeys of 4 hours or more. A 2021 Cochrane review pooled 12 randomized trials on flights longer than 5 hours. Asymptomatic deep-vein thrombosis detected only by screening was less common with graduated compression stockings: odds ratio OR 0.10 (95% CI 0.04–0.25). This is a rare outcome, so odds ratios approximate relative risks. Among 2,637 participants, 50 cases were detected: 3 among stocking users and 47 among non-users. No participant died or developed pulmonary embolism or symptomatic thrombosis. The trials therefore cannot establish whether stockings prevent these serious outcomes. WHO advises asking a doctor before travel if you have questions about prevention. NHS lists pain and swelling in one leg as DVT symptoms. If suspected, seek a doctor promptly; they arrange hospital ultrasound within 24 hours. If leg pain and swelling also come with breathlessness or chest pain, call emergency services.

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：「Individuals with questions regarding prevention of VTE should consult their physicians before travelling」. https://www.who.int/news/item/29-06-2007-study-results-released-on-travel-and-blood-clots ; NHS. DVT (deep vein thrombosis)：「throbbing pain in 1 leg (rarely both legs), usually in the calf or thigh」「swelling in 1 leg (rarely both legs)」「Ask for an urgent GP appointment or get help from NHS 111 if: you think you have DVT (deep vein thrombosis)」「Call 999 or go to A&E if: You have symptoms of DVT (deep vein thrombosis), such as pain and swelling, and: feel short of breath」「have chest pain」「you should be referred to hospital within 24 hours for an ultrasound scan」「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

Swelling in both legs is more often a problem involving heart, liver, kidney or venous function. Sudden swelling in one leg, making it noticeably larger than the other, is typical of deep-vein thrombosis. Risk factors include long car or plane journeys, bed rest after surgery or fractures, a cast, pregnancy and the period around childbirth, cancer, and oral contraceptives. A clot breaking off and blocking a pulmonary artery causes pulmonary embolism: sudden breathlessness, chest pain worse on deep inhalation, a fast heartbeat, dizziness or coughing up blood. Call 120 immediately at this point. The 17.4% figure is mortality among confirmed pulmonary embolism cases; it is not the risk associated with any swollen leg, so do not interpret it that way. The WHO study did not test preventive measures. It mentioned flexing the ankles and moving the calves while sitting, avoiding tight clothes during travel, and asking a doctor before departure if in doubt. The stocking trials included high-risk passengers but measured only small asymptomatic clots. Ask a doctor first about wearing stockings if you have high-risk factors.