What causes dizziness and fainting during flights?

Why People Feel Dizzy and Pass Out During Flights

Dizziness and fainting during flights result primarily from two mechanisms: moderate hypoxia due to reduced cabin pressure (equivalent to 5,000-8,000 feet altitude) causing oxygen saturation to drop to 90-93%, and dehydration from extremely low cabin humidity (5-20%) leading to volume depletion and orthostatic hypotension. 1

Primary Mechanisms

Hypoxia-Related Effects

Cabin pressurization creates an environment equivalent to 2,400 meters (8,000 feet) altitude, causing oxygen saturation to fall from normal levels to 90-93% in healthy individuals. 1 This reduction triggers several physiological responses:

• Changes in brain glucose metabolism occur in specific regions, with compensatory increases in cerebral blood flow to maintain oxygen delivery. 1 Even in healthy subjects, these changes commonly produce dizziness, nausea, and headache. 1

• The hypoxic stress acts as a cardiac stimulant, increasing heart rate and cardiac output. 1 However, this compensation can be inadequate in individuals with pre-existing conditions or reduced physiological reserves. 2

• Light physical activity during flight (such as walking to the lavatory) significantly worsens hypoxemia and can precipitate syncopal episodes. 1

Dehydration and Volume Depletion

Cabin humidity drops to 5-20%, causing fluid loss that becomes symptomatic after 3-4 hours of exposure. 1 The dehydration mechanism is multifactorial:

• Low humidity and cooled air increase resting ventilatory water losses by approximately 200 ml per hour. 1

• Immobilization for 4 hours decreases plasma volume by approximately 6%, with blood pooling in the lower extremities and fluid shifting to interstitial spaces in the legs. 1

• Alcohol and caffeine consumption during flight promotes diuresis and further increases fluid loss. 1

• Dehydration leads to orthostatic hypotension, which is the most common cause of in-flight syncope. 3 Signs include postural dizziness, increased heart rate, muscle weakness, and confusion. 1

Contributing Environmental Stressors

Pre-flight stress (airport commotion, rushing to gates, carrying luggage) increases physical and mental exertion, predisposing passengers to medical events before boarding. 1, 2

Acceleration forces, vibration, turbulence, and noise during flight add cumulative physiological stress. 2 These factors are particularly problematic for individuals with reduced physiological reserves. 2

High-Risk Populations

Certain groups face amplified risk:

• Individuals taking diuretics, alpha-blockers, or RAAS inhibitors are at increased risk due to medication-induced volume depletion. 1

• Passengers with pre-existing cardiovascular disease, respiratory conditions, or autonomic dysfunction have baseline impairments that are exacerbated by flight conditions. 1, 4

• Older adults and those with cognitive impairment show more severe symptoms due to baseline alterations in cerebral blood flow and autonomic function. 1

Clinical Presentation

Symptoms range from mild dizziness and fatigue to complete loss of consciousness (syncope). 1, 3 The spectrum includes:

• Lightheadedness, particularly with position changes 1
• Nausea and headache 1
• Chest tightness and dyspnea 1
• Confusion and disorientation 1
• Frank syncope requiring medical intervention 3

Prevention Strategies

Passengers should increase fluid intake by 0.5-1 liter per day during long-haul flights and avoid alcohol or excessive coffee consumption. 1 Additional measures include:

• Regular movement and ambulation during flight to prevent blood pooling. 1
• Avoiding sudden position changes, particularly when standing from prolonged sitting. 1
• Pre-planned assistance with luggage and wheelchair transport at airports for high-risk individuals. 1

Important Caveats

Postural hypotension from dehydration is the most common mechanism, but hypoxia-induced syncope ("airline syncope") can occur even in individuals without underlying autonomic dysfunction. 3 The two mechanisms often act synergistically, making it difficult to isolate a single cause in any given episode. 1, 3

Healthy passengers normally tolerate flight stresses well, but those with pre-existing medical conditions and reduced physiological reserves are at substantially higher risk. 2 The combination of hypoxia, dehydration, immobility, and environmental stress creates a perfect storm for cardiovascular compromise. 1, 3