A stroke occurs when blood flow to a part of the brain is interrupted, either by a blockage (ischemic stroke) or by a burst blood vessel (hemorrhagic stroke). Air travel is generally safe for the majority of people, but flying introduces physiological stressors that can increase the risk of an ischemic stroke in certain vulnerable individuals. The concern is not the act of flying itself, but the combination of environmental changes and prolonged physical inactivity inherent to long-haul journeys. This elevated risk stems primarily from factors that encourage the formation of blood clots, which can then travel to the brain.
The In-Flight Environmental Factors
Commercial aircraft pressurize the cabin to simulate an altitude equivalent to 6,000 to 8,000 feet above sea level. This reduced air pressure means less oxygen is available, leading to mild hypoxia (a reduction in blood oxygen saturation). For healthy travelers, this mild hypoxia is well-tolerated and does not pose a significant stroke risk. This environmental change is primarily a concern for passengers who already have severe underlying cardiopulmonary conditions, such as chronic obstructive pulmonary disease or poorly controlled heart failure.
Another significant factor is the extremely low humidity inside the cabin, which often drops to between 10 and 20 percent. This dry environment promotes dehydration, which in turn can increase the concentration and viscosity of the blood. Thicker blood flows less efficiently and is more prone to clotting, which compounds the risk presented by immobility.
Understanding Immobility and Clot Risk
The most significant factor linking air travel to stroke risk is prolonged immobility, particularly during flights lasting four hours or more. Remaining sedentary for long periods allows blood to pool in the deep veins of the lower legs, a phenomenon called venous stasis. This stagnant blood flow encourages the formation of a clot, known as a Deep Vein Thrombosis (DVT).
While the absolute risk for a DVT remains low for the general population, the consequences can be severe. If a clot breaks loose from the leg vein, it becomes an embolus and typically travels to the lungs, causing a potentially fatal pulmonary embolism. In some cases, the clot can travel to the brain, causing an ischemic stroke through a process called a paradoxical embolism.
This occurs when the embolus passes from the venous circulation (the right side of the heart) to the arterial circulation (the left side of the heart) without first passing through the lungs. The clot bypasses the lungs by passing through a small, flap-like opening in the heart called a Patent Foramen Ovale (PFO). A PFO is a residual opening from fetal development present in about 25% of the general population. When the clot crosses this opening and travels to the brain, it blocks a cerebral artery, resulting in a stroke.
Identifying High-Risk Passengers and Precautions
A small subset of the traveling public is at a higher risk for flight-related clotting events. Individuals with a history of DVT, pulmonary embolism, or a previous stroke should exercise extra caution. Other groups include those with severe heart conditions like atrial fibrillation, known blood clotting disorders, obesity, and anyone who has undergone recent major surgery, especially orthopedic procedures.
Taking preventative measures can mitigate the risk for all travelers. The most effective action is frequent movement, including walking the aisle every one to two hours and performing in-seat exercises like ankle circles and calf pumps. Staying hydrated with water or electrolyte beverages is important, while avoiding excessive consumption of alcohol and caffeine, which contribute to dehydration.
High-risk individuals should consult their doctor about wearing properly fitted, graduated compression stockings, typically rated at 15 to 30 mmHg. For those at the highest risk, a physician may recommend prophylactic anticoagulant medication, such as low molecular weight heparin, especially for long-haul flights.