**2. Epidemiology**

The clinical relevance of these asymptomatic thromboses is questionable, but they may be the origin for the manifestation of deep vein thrombosis. So far, there is no prospective, controlled study with a sufficient number of cases (adequate statistical power) that proves an evidence-based association between air travel and the risk of venous thromboembolism (VTE). In order to demonstrate a significantly increased risk, a study including around 1 million passengers (+ controls) would be necessary [13].

The risk for VTE mainly depends on the flight duration, the number of flights within 3–4 weeks, and risk factors of each individual predisposing to VTE. According to meta-analyses from case-control studies, long-haul flights increase the risk of VTE by two to four times within the first 4 weeks. For every 2 h of extended flight time, the VTE risk may increase by 26%. The risk of VTE is not increased in healthy passengers without risk factors and a flight duration of less than 3–4 h. It should be taken into account in this context that the basic risk for VTE in healthy women under 35 years without hormone intake is 1–2/10,000/ years and increases fourfold after the age of 40 [13–16]. Young women (<30 years) are up to three times more likely to experience VTE than young men. An observational study from France analyzed 116 pulmonary embolisms (PE) over a period of 13 years, which occurred immediately after landing. With approximately the same gender distribution of passengers, 78% of all PE affected women [13–16]. Overall, the incidence of PE was 0.61/1,000,000 female passengers (0.2/1,000,000 male passengers). With a flight distance of more than 10,000 km, this risk rises to 7.2/1,000,000 female passengers. According to another study to 4.8/1,000,000 female passengers with a flight duration over 12 h, the total rate of VTE in healthy women without oral contraceptives is estimated 1/5000 regardless of the flight duration [13–16].
