The reality is that deep vein thrombosis (DVT) and pulmonary embolism (PE) affect up to half a million people in Europe every year. Both these conditions are described together as venous thromboembolism (VTE/blood clot). VTE is also the single, most common cause of preventable deaths associated in hospitals; furthermore, up to 50% of individuals with DVT develop the postthrombotic syndrome (PTS), with its significant impact on quality of life and significant costs for health departments. It is estimated that 10 million cases of VTE occur every year worldwide. The longest existing flight is the Singapore Airlines service from Singapore to Newark, New Jersey, which has a duration of 19 hours.

Immobility is amongst the most important recognised risk factors for VTE. Long-haul travel is associated with individuals (passengers, pilots, crews) who find themselves within a forced state of relative immobility for prolonged periods and now with the emergence of ultra-long-haul flights, even longer. Importantly, VTE prevention is key, which is performed by providing either anti-VTE  medicines or mechanical compression stockings to individuals deemed at risk of developing this condition. Flight length >4 hours is defined as long-haul.

DVT, which, in 30% of individuals, can progress to the development of a PE. PTS develops in 50% of patients with DVT and is characterised by chronic venous disease, swelling, skin changes and in the most severe cases, venous leg ulceration.

The 2018 International Air Transport Association (IATA) forecast predicts that, by 2037, air passenger numbers could double to 8.2 billion, which will be associated with a shift in travel toward the Asia-Pacific region, which will drive the largest growth of new passengers and will likely result in a larger number of individuals travelling on long-haul flights.

Prevalence: Retrospective data has estimated the overall incidence of pulmonary embolism as 0.39 per one million passengers; for flights up to 8 hours, the incidence was 0.25, while, for flights longer than 8 hours, this increased to 1.65 per one million passengers. A meta-analysis exploring the relationship between travel >8 hours and VTE events revealed a relative risk for VTE of 2.8, with an 18% additional risk for each 2-hour increase in travel duration by any mode; interestingly, the increased relative risk for air travel was 26% for every 2 additional hours, suggesting that flying conferred an increased risk of VTE compared with other forms of transportation.

VTE after flight realities: There is evidence that the increase in VTE risk following a long-haul flight can last for a number of weeks. A study of 8755 participants exposed to flights of at least 4 hours or more recorded a VTE incidence of 1 event per 4656 long-haul flights, of which 42% occurred in the first 8 weeks following travel, with particularly high rates in the first 2 weeks.