title: The absolute risk of venous thrombosis after air travel: a cohort study of 8,755 employees of international organisations.
abstract: BACKGROUND
  The risk of venous thrombosis is approximately 2- to 4-fold increased after air travel, but the absolute risk is unknown. The objective of this study was to assess the absolute risk of venous thrombosis after air travel.
  
  
  RESULTS
  We conducted a cohort study among employees of large international companies and organisations, who were followed between 1 January 2000 and 31 December 2005. The occurrence of symptomatic venous thrombosis was linked to exposure to air travel, as assessed by travel records provided by the companies and organisations. A long-haul flight was defined as a flight of at least 4 h and participants were considered exposed for a postflight period of 8 wk. A total of 8,755 employees were followed during a total follow-up time of 38,910 person-years (PY). The total time employees were exposed to a long-haul flight was 6,872 PY. In the follow-up period, 53 thromboses occurred, 22 of which within 8 wk of a long-haul flight, yielding an incidence rate of 3.2/1,000 PY, as compared to 1.0/1,000 PY in individuals not exposed to air travel (incidence rate ratio 3.2, 95% confidence interval 1.8-5.6). This rate was equivalent to a risk of one event per 4,656 long-haul flights. The risk increased with exposure to more flights within a short time frame and with increasing duration of flights. The incidence was highest in the first 2 wk after travel and gradually decreased to baseline after 8 wk. The risk was particularly high in employees under age 30 y, women who used oral contraceptives, and individuals who were particularly short, tall, or overweight.
  
  
  CONCLUSIONS
  The risk of symptomatic venous thrombosis after air travel is moderately increased on average, and rises with increasing exposure and in high-risk groups.
authors: Department of Clinical Epidemiology, Leiden University Medical Center, Leiden, The Netherlands.
authors: Saskia
authors: S
authors: Kuipers
authors: Suzanne C
authors: SC
authors: Cannegieter
authors: Saskia
authors: S
authors: Middeldorp
authors: Luc
authors: L
authors: Robyn
authors: Harry R
authors: HR
authors: Büller
authors: Frits R
authors: FR
authors: Rosendaal
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000401:Aircraft / Q000706:statistics & numerical data; D015331:Cohort Studies; D003132:Commerce / Q000706:statistics & numerical data / Q000639:trends; D005260:Female; D006801:Humans; D038622:Internationality; D008297:Male; D008875:Middle Aged; D016272:Occupational Health / Q000706:statistics & numerical data; D012307:Risk Factors; D013997:Time Factors; D014195:Travel / Q000706:statistics & numerical data / Q000639:trends; D020246:Venous Thrombosis / Q000453:epidemiology* / Q000209:etiology*