What are the odds of a serious blood clot after a long-haul flight?
Evidence quality 4.75/5
Eight-dimension review score against the quality rubric . Each dimension scored 1–5.
- D1 Source grounding
- 5/5
- D2 Source authority
- 5/5
- D3 Arithmetic
- 5/5
- D4 Uncertainty
- 4/5
- D5 Scope
- 5/5
- D6 Prose
- 5/5
- D7 Perception honesty
- 4/5
- D8 Caveat completeness
- 5/5
Lifetime probability · lifetime, activity-specific
1 in 4,651
0.02% lifetime chance
Most people overestimate this.
range 1 in 10,000 to 1 in 2,000
● your factors — click this risk ▾ to reveal
≈ As likely as
Perceived
Blood clots from flying occupy a reliable slot in the pre-trip anxiety rotation, somewhere between deep-vein dread and the impulse to buy compression stockings at the airport pharmacy. The fear is specific enough to drive a small consumer market — flight socks, aspirin before boarding, aisle-seat upgrades “so I can walk around” — yet most travelers would struggle to put a number on the actual risk. The intuitive sense is that it is meaningfully dangerous, probably somewhere around 1 in 100 to 1 in 500 per long flight, which overshoots the epidemiology by one to two orders of magnitude.
Rough estimate: most travelers guess somewhere in the 1-in-100 to 1-in-500 range per long flight
Source: editorial intuition, not polled
Actual
~1 in 4,656 per long-haul flight (>4 h)
employees of international organisations taking flights >4 hours (Kuipers et al. cohort)
Show derivation
The headline figure comes directly from Kuipers et al. (2007): one symptomatic VTE event per 4,656 long-haul flights (>4 hours) in a cohort of 8,755 employees of international organisations followed for six years. This is an absolute incidence figure for a single flight, not a lifetime accumulation, so normalized.lifetime_us_adult here represents the per-flight probability (~0.0215%) rather than a conventional US-adult lifetime figure. The scope is set to activity_specific_lifetime accordingly. The Chandra et al. (2009) meta-analysis found a pooled relative risk of 2.0 (95% CI 1.5–2.7) for VTE in travelers vs non-travelers, with a 26% increase per additional 2 hours of air travel, consistent with the Kuipers absolute rate. Fatal pulmonary embolism from a single long-haul flight is estimated at roughly 1 in 1,000,000 based on the observation that PE accounts for roughly 10–20% of VTE events and case fatality for PE in otherwise healthy travelers is low.
Caveats: The headline ~1-in-5,000 figure represents any symptomatic venous thromboembolis…
The headline ~1-in-5,000 figure represents any symptomatic venous thromboembolism (DVT or PE) within 8 weeks of a single long-haul flight (>4 hours), not death. Most events are deep vein thrombosis, which is treatable and rarely fatal. Pulmonary embolism, the dangerous complication, accounts for a minority of these events and is estimated at roughly 1 in 20,000–50,000 per flight; fatal PE at roughly 1 in 1,000,000 per flight. The Kuipers cohort consists of employees of international organisations who are younger and healthier than the general population, which may underestimate risk for older or less healthy travelers. Conversely, this population flies frequently, and the study found that multiple flights in short timeframes elevate risk further. The personal factor multipliers are drawn from the Cannegieter MEGA case-control study and represent odds ratios for the combined exposure (travel + factor), not clean multiplicative interactions; they are order-of-magnitude guides, not precise adjustments. Current clinical guidelines (ACCP, NICE) do not recommend aspirin prophylaxis for air travel; graduated compression stockings are supported for high-risk travelers only.
Risks at similar odds
Other risks with roughly the same likelihood — useful for calibration.
Child bike trailer
What are the odds of serious injury to a child riding in a towed bicycle trailer?
Child rear bike seat
What are the odds of serious injury to a child riding in a rear-mounted bike seat?
Child without restraint
How much more likely is a child to die in a car crash without an appropriate child restraint?
Teen road-crash death
How likely is a teenager (15–19) to die in a road-traffic crash during those years?
Recently viewed on this device
Stored locally — clear anytime.
Pick challenger
The best prospective estimate of blood clot risk from a long-haul flight comes from Kuipers et al. (2007), who tracked 8,755 employees of international organisations over six years and found one symptomatic VTE event per 4,656 flights longer than four hours. The Chandra et al. meta-analysis (2009) corroborates this from the relative-risk side: travelers face roughly twice the VTE risk of non-travelers, with a 26% escalation for every additional two hours in the air. For context, the per-flight VTE risk (~1 in 5,000) is about 3,000 times higher than the per-flight risk of dying in a plane crash — but the VTE itself is overwhelmingly treatable DVT, not the fatal pulmonary embolism that the fear is really about. Fatal PE from a single long-haul flight sits around 1 in 1,000,000, which is comparable to the plane crash number.
The interesting part of this fear is less the base rate and more the interaction effects. The Cannegieter MEGA study (2006) found that oral contraceptive use combined with travel of more than four hours produced an odds ratio above 20 for venous thrombosis — roughly a fourteen-fold increase over the travel-only baseline. Obesity pushed the combined OR to 9.9. Factor V Leiden carriers who traveled had an OR of 8.1. The base rate is low enough that even these multipliers leave most individual flights in the “very small risk” territory the CDC describes, but a woman on combined oral contraceptives taking a twelve-hour flight after recent knee surgery is in a genuinely different risk category from the median healthy traveler, and that subgroup is where clinical prophylaxis guidelines actually apply.
What the fear drives in practice is mostly misallocated precaution. Compression stockings have moderate evidence for high-risk travelers (ACCP guidelines), and walking or calf exercises during the flight are sensible if unproven at the population level. But aspirin before a flight — the single most common self-prescribed “precaution” — is not recommended by any current clinical guideline for travel-related VTE prophylaxis. The fear is overrated for the vast majority of flyers and underrated for the small, identifiable group where it actually matters.
Claim ledger
Every number below is what each source reported, with the verbatim quote we relied on and how we arrived at our figure. Click any link to verify directly.
-
[1] PLoS Medicine (Kuipers S, Cannegieter SC, Middeldorp S, Robyn L, Büller HR, Rosendaal FR) — The absolute risk of venous thrombosis after air travel: a cohort study of 8,755 employees of international organisations
The absolute risk of venous thrombosis after air travel: a cohort study of 8,755 employees of international organisations- Statistic
One VTE event per 4,656 long-haul flights; incidence rate 3.2/1,000 person-years after long-haul flights vs 1.0/1,000 person-years in unexposed periods; risk ratio 3.2 (95% CI 1.8-5.6)- Excerpt
“"Incidence rate of 3.2/1,000 PY after long-haul flights versus 1.0/1,000 PY in individuals not exposed. One event per 4,656 long-haul flights." ”
- Source data from
- 2007-09-01
- Accessed
- 2026-04-11 · archived copy
- Calculation
- Kuipers tracked 8,755 employees of international organisations (UN system, World Bank, etc.) from 2000-2005, accumulating 6,872 person-years of long-haul-flight-exposed time. 22 VTE events occurred within 8 weeks of a long-haul flight. The “1 per 4,656 flights” figure is the primary absolute risk estimate and maps directly to our native value. The cohort is younger and healthier than the general population (employed, medically cleared for international travel), so this may slightly underestimate risk for the general flying public but is the best prospective absolute risk figure available.
- Independence
- Kuipers is the primary prospective cohort study on flight-associated VTE. Fully independent of the Chandra meta-analysis (which pooled other studies but not Kuipers itself) and of CDC's secondary guidance; shares Leiden-group co-authors with the Cannegieter MEGA case-control study referenced in the personal-factor multipliers, so treat the two Leiden-pipeline estimates as methodologically linked.
-
[2] Annals of Internal Medicine (Chandra D, Parisini E, Mozaffarian D) — Meta-analysis: travel and risk for venous thromboembolism
Meta-analysis: travel and risk for venous thromboembolism- Statistic
Pooled relative risk for VTE in travelers 2.0 (95% CI 1.5-2.7); 26% higher risk for every 2 hours of air travel; 18% higher risk per 2-hour increase in travel by any mode- Excerpt
“"Overall pooled relative risk for VTE in travelers was 2.0 (95% CI, 1.5 to 2.7). 26% higher risk for every 2 hours of air travel." ”
- Source data from
- 2009-08-04
- Accessed
- 2026-04-11 · archived copy
- Calculation
- Chandra et al. pooled 14 studies (4,055 VTE cases). The 2.0 relative risk is for any travel >4 hours vs no travel; when corrected for selection bias in control groups, the estimate rises to 2.8 (CI 2.2–3.7). The dose-response gradient (26% per 2h of air travel) supports the biological plausibility of the Kuipers absolute risk figure and anchors the “risk rises with duration” framing. This is a relative risk, not an absolute risk; combined with background VTE incidence of ~1–2 per 1,000 person-years, it is consistent with the Kuipers 1-in-4,656-flights figure.
- Independence
- Chandra et al. meta-analysis includes the Cannegieter MEGA study data but not the Kuipers cohort study (published in the same year). The two primary sources here are therefore largely independent on the population data, though they share co-authors from the Leiden group.
-
[3] US Centers for Disease Control and Prevention (CDC) — Understanding Your Risk for Blood Clots with Travel
Understanding Your Risk for Blood Clots with Travel- Statistic
More than 300 million people travel on long-distance flights each year; risk of developing a blood clot is generally very small; most people who develop travel-associated blood clots have one or more other risk factors- Excerpt
“"Even if you travel a long distance, the risk of developing a blood clot is generally very small. Most people who develop travel-associated blood clots have one or more other risks for blood clots." ”
- Source data from
- 2024-05-14
- Accessed
- 2026-04-11 · archived copy
- Calculation
- CDC does not publish a specific per-flight incidence figure but corroborates the “generally very small” framing and confirms that the risk-factor-dependent subgroup story is the dominant clinical concern. The 300-million-flights-per-year denominator is useful context for the absolute risk figure from Kuipers. Used as authoritative corroboration, not as the primary quantitative source.
- Independence
- CDC guidance is editorially independent of the Leiden group studies but cites the same underlying literature base.