Several medical conditions can make commercial flying dangerous or outright inadvisable, ranging from recent heart attacks and untreated pneumothorax to eye surgery involving gas bubbles and advanced pregnancy. The core issue is the airplane cabin itself: at cruising altitude, cabin pressure drops to the equivalent of sitting on a mountain roughly 6,000 to 8,000 feet high, which lowers blood oxygen and causes trapped gases in the body to expand by about 35 percent. For most people, these shifts are harmless. For people with certain conditions, they can trigger a genuine medical emergency at 35,000 feet, where getting help is limited and landing quickly is expensive and slow.
Why the Cabin Environment Creates Risk
A commercial aircraft cabin is pressurized, but not to sea-level pressure. The standard is roughly equivalent to 6,000 to 8,000 feet of altitude. That means two things happen to your body the moment the plane climbs. First, the oxygen level in your blood drops mildly, because the air is thinner than what you breathe on the ground. Second, any gas trapped inside a closed space in your body expands as the surrounding pressure falls. These two effects, mild hypoxia and gas expansion, are the underlying reason almost every flying restriction exists.1PubMed Central. Medical issues associated with commercial flights For a healthy person, the body adapts without trouble. For someone whose heart, lungs, or other organs are already compromised, that small shift can push things past a tipping point.
Heart and Circulatory Conditions
Cardiovascular problems are among the most serious reasons to postpone or avoid flying. If you have had an uncomplicated heart attack, most guidelines say you can fly after about 10 days, as long as you are symptom-free. But if the heart attack involved complications such as heart failure or abnormal heart rhythms, the recommended wait stretches to two to six weeks, and you should be properly evaluated before booking a flight.2PubMed Central. Cardiovascular disease and airline travel Flying is flatly contraindicated if you have unstable angina, a recent complicated heart attack, decompensated heart failure, severe untreated high blood pressure, or complex ventricular arrhythmias that haven’t been stabilized.2PubMed Central. Cardiovascular disease and airline travel
The reason is straightforward. When blood oxygen dips even modestly, the heart has to work harder to push enough oxygenated blood around the body. A healthy heart handles the extra workload without complaint. A heart that has recently been damaged or is failing may not, and the result can be chest pain, dangerous rhythm disturbances, or cardiac arrest. Add in the dehydration and prolonged immobility of a long flight, and the cardiovascular stress compounds. Acute cardiac emergencies in the air are one of the most common reasons flights divert, accounting for more than a quarter of diversions in a large recent analysis.3PubMed Central. In-Flight Medical Events on Commercial Airline Flights
Lung and Breathing Problems
Respiratory conditions are the other major category where the cabin environment causes trouble. The mild hypoxia of flight is tolerable for healthy lungs but can tip someone with chronic obstructive pulmonary disease (COPD), severe asthma, pulmonary fibrosis, or other chronic lung conditions into dangerously low oxygen levels. Some people with chronic lung disease undergo a pre-flight “hypoxia altitude simulation test,” which essentially has them breathe low-oxygen air for 20 minutes to predict whether they will need supplemental oxygen on board.4PubMed Central. Predicting the need for supplemental oxygen during airline flight in patients with chronic pulmonary disease: a comparison of predictive equations and altitude simulation The test doesn’t always predict whether someone will actually feel symptoms during the flight, but it helps doctors decide who needs in-flight oxygen.5PubMed. COPD and air travel: does hypoxia-altitude simulation testing predict in-flight respiratory symptoms?
Pneumothorax, a condition where air leaks into the space around the lung and partially collapses it, is a particularly clear-cut restriction. Gas trapped in the pleural space expands as the cabin pressure drops, potentially worsening the collapse. At a typical cabin altitude of 8,000 feet, trapped gas expands by roughly 35 percent, so a pneumothorax occupying 10 percent of the chest could swell to about 13.5 percent.6PubMed Central. When Is It Safe to Fly? Early Air Travel After Small Traumatic Pneumothorax In a small, resolved pneumothorax in an otherwise healthy person, that expansion may be clinically insignificant. But people with cystic lung diseases, a recent pneumothorax that hasn’t fully healed, or recent chest surgery face real danger, because the expansion can re-collapse the lung or turn a small leak into a big one.7CHEST. Air Travel and Pneumothorax Most physicians advise waiting until imaging confirms the pneumothorax has completely resolved before flying, though exactly how long that takes varies by case.
After Eye Surgery With Gas Bubbles
This is one of the less intuitive restrictions, but it is taken extremely seriously. Certain retinal surgeries involve placing a gas bubble inside the eye to press the retina back into place while it heals. Common gases used include sulfur hexafluoride and perfluoropropane. If you fly before that bubble has fully absorbed, the reduced cabin pressure causes it to expand, which sharply raises the pressure inside the eye. Theoretical modeling shows that this creates a two-fold risk: at lower altitudes the bubble shrinks and can cause dangerously low eye pressure and retinal re-detachment, while at high cabin altitudes the expanding bubble and accumulated fluid drive pressure spikes even without exceeding the altitude of the original surgery.8PubMed. Patients With Intravitreal Gas Bubbles at Risk of High Intraocular Pressure Without Exceeding Elevation of Surgery: Theoretical Analysis
The consequences of ignoring this restriction can be severe. There are documented cases of patients losing vision after exposure to pressure changes while a gas bubble was still present in the eye.9PubMed. Use of nitrous oxide causing severe visual loss 37 days after retinal surgery The waiting period depends on which gas was used: sulfur hexafluoride bubbles typically absorb in a couple of weeks, while perfluoropropane can linger for two months or more. Your surgeon will tell you specifically when it is safe. Until then, flying is a hard no, and so is traveling to high-altitude locations by car.
Ear and Sinus Conditions
Anyone who has flown with a head cold knows the painful ear pressure during descent. That discomfort is a mild version of what can happen to people with conditions that block the Eustachian tube, which connects the middle ear to the back of the throat and allows pressure to equalize. During descent, the cabin pressure rises, and the middle ear needs to let air in through the Eustachian tube to keep up. When the tube is blocked by severe congestion, infection, swelling, or structural abnormality, a pressure difference builds across the eardrum. A difference of around 60 mmHg causes significant pain, and at differentials between 100 and 500 mmHg the eardrum can actually rupture.10PubMed Central. Surgical and Conservative Management in Otitic Barotrauma: A Retrospective Cohort Study
People with acute ear infections, recent ear surgery, severe sinusitis, or a recently repaired eardrum are commonly told not to fly until the condition has healed. Chronic problems with Eustachian tube dysfunction also make flying consistently miserable and occasionally harmful. If you must fly with mild congestion, a decongestant spray used before descent can help the tube open, but if there is active middle-ear disease, the safest choice is to stay on the ground.
Sickle Cell Disease
Sickle cell disease deserves its own discussion because the cabin environment hits it from a specific angle. The mild drop in blood oxygen during flight can trigger red blood cells with the sickle hemoglobin to change shape and clump together, blocking small blood vessels. This vaso-occlusive crisis can cause sudden, severe pain and organ damage. Research suggests that painful crises may occur in up to about 9 percent of sickle cell patients during air travel.11PubMed Central. The Traveler with Sickle Cell Disease Flying is not absolutely forbidden for people with sickle cell disease, but it requires careful preparation: staying well hydrated, moving around the cabin regularly, and sometimes using supplemental oxygen. People who have frequent crises or whose disease is poorly controlled should discuss flying with their hematologist beforehand.
Pregnancy
For an uncomplicated pregnancy, flying is generally safe. A large retrospective study of more than 284,000 pregnancies found no evidence that air travel during pregnancy was associated with preterm birth or lower birth weight, which lines up with the American College of Obstetricians and Gynecologists’ stance that routine air travel is not restricted for pregnant women.12PubMed Central. Air travel during pregnancy and the risk of adverse pregnancy outcomes as gestational age and weight at birth: A retrospective study among 284,069 women in Israel between the years 2000 to 2016 Most airlines do set their own cutoffs, typically allowing travel up to 36 weeks for a single pregnancy and 32 weeks for twins, mainly because airlines do not want to deal with an onboard delivery.
The situation changes for pregnancies with complications. Women at elevated risk for preterm labor, those with placental abnormalities like placenta previa, and pregnant women with underlying respiratory or cardiac disease are advised not to fly.13PubMed Central. Is air travel in pregnancy safe? The reduced oxygen and lower cabin humidity add physiological stress that a complicated pregnancy may not tolerate well. If your pregnancy involves any of these risk factors, your obstetrician is the right person to make the call.
Recent Scuba Diving
Flying too soon after scuba diving is one of the more well-known restrictions among divers, but many casual vacationers aren’t aware of it. When you dive, nitrogen dissolves into your tissues under the higher pressure of the water. If you then fly and the cabin pressure drops, that dissolved nitrogen can come out of solution and form bubbles in your blood and tissues, which is decompression sickness. It can cause joint pain, nerve damage, and in severe cases, stroke-like symptoms or death.
Guidelines established by the Undersea and Hyperbaric Medical Society recommend that recreational divers who have done standard no-decompression dives totaling less than two hours of dive time wait at least 12 hours before flying. After multiple days of diving, the recommended wait is 24 hours. If dives required decompression stops, the minimum wait is 24 hours, and 48 hours is preferred.14PubMed. Flying after diving guidelines: a review Even with a correct 24-hour wait, some individuals appear more susceptible to bubble formation than others. An in-flight echocardiography study found that certain divers who consistently produced high bubble grades after dives showed similar bubble grades during flight despite following the standard waiting period.15PubMed. Flying after diving: in-flight echocardiography after a scuba diving week If you are someone who has experienced decompression sickness symptoms after diving before, you may need longer surface intervals before flying than the standard guidelines suggest.
Casts, Fractures, and Immobilization
A freshly applied plaster cast combined with a flight creates a specific risk: as the limb swells during the flight (a combination of immobility, low cabin pressure, and the inflammatory process from the fracture itself), a rigid cast can become a tight compartment. Most airlines recommend delaying the flight at least 24 to 48 hours after a full cast is applied, and some require the cast to be split (bivalved) if applied within 48 hours of travel.16PubMed. My child’s leg is in a plaster. Can they fly? If the flight is truly urgent, bivalving the cast before boarding is the standard safety measure. This applies to children and adults alike, though pediatric casts get extra scrutiny because children are less able to articulate that something feels wrong.
Beyond the cast itself, people with recent leg fractures or surgeries face the same deep vein thrombosis risk as any immobilized passenger, compounded by the injury. Compression stockings, adequate hydration, and moving whatever limbs you can during the flight are all standard precautions.
Premature Infants and Young Children With Lung Disease
Infants who were born prematurely and had neonatal lung disease are quietly one of the higher-risk groups for flying. Their lungs may be underdeveloped and have less reserve to handle the mild hypoxia of cabin altitude. A retrospective study of preterm infants referred for fitness-to-fly testing found that 81 percent of them desaturated below safe levels during a simulated altitude challenge and needed supplemental oxygen prescribed for the flight. Younger infants were at far greater risk: children who failed the test were significantly younger than those who passed, with a median corrected age near birth compared to nearly 13 months for those who passed.17PubMed Central. Pre-flight testing of preterm infants with neonatal lung disease: a retrospective review If your child was born prematurely and is under a year old, a pre-flight assessment with a pediatrician is worth the effort, even if the baby seems perfectly healthy and is off supplemental oxygen at home.
Psychiatric Conditions and Behavioral Safety
Physical health conditions get most of the attention, but mental health conditions can also be grounds for not flying, or at least for careful planning. An acutely psychotic or severely agitated person poses safety risks to themselves and other passengers in the confined environment of an aircraft, and airlines can and do refuse boarding if someone appears unable to comply with safety instructions or behaves unpredictably. Beyond safety, the stress of travel itself, including disrupted sleep, unfamiliar environments, crossing time zones, and crowds, can destabilize psychiatric conditions that were marginally controlled on the ground.18PubMed Central. The most vulnerable travelers: patients with mental disorders People with well-managed anxiety, depression, or other psychiatric conditions generally fly without problems. The concern is with acute, unstabilized states where behavior could become dangerous or unmanageable in the air.
How Common Are In-Flight Medical Emergencies
Despite all these restrictions, most people fly without incident. A 2025 analysis of nearly 78,000 in-flight medical events found an overall incidence of about 39 events per million passenger boardings, or roughly one event for every 212 flights. Aircraft diversions, the most disruptive outcome, happened in fewer than 2 percent of those cases. When diversions did occur, neurological events like suspected strokes were the most frequent trigger, followed by cardiovascular emergencies.3PubMed Central. In-Flight Medical Events on Commercial Airline Flights A separate systematic review covering roughly 1.5 billion passengers estimated an overall in-flight medical emergency rate of about 18 per million passengers and an all-cause death rate of about 0.2 per million passengers. The most common events were fainting, gastrointestinal complaints, and respiratory or neurological episodes. Emergency diversions, when they occurred, cost airlines anywhere from $15,000 to nearly $900,000 per landing.19The American Journal of Emergency Medicine. The global incidence of in-flight medical emergencies: A systematic review and meta-analysis of approximately 1.5 billion airline passengers
These numbers are reassuring for the average traveler, but they also illustrate why airlines and doctors take pre-existing conditions seriously. A suspected stroke at 38,000 feet over the ocean, hours from the nearest hospital, is a fundamentally different medical situation than one that happens on the ground. The restrictions exist not because flying is inherently dangerous but because the cabin environment removes your safety net: lower oxygen, no emergency room, and no easy way to stop the ride.
Conditions People Worry About That Usually Aren’t a Problem
Not everything that feels like it should be a flying restriction actually is one. Well-controlled diabetes, for instance, does not prevent you from flying, though you should carry your medications in your hand luggage and adjust insulin timing for time-zone shifts. Epilepsy that is stable and well managed is similarly fine, though the fatigue and disrupted sleep of long-haul travel can lower seizure thresholds in some people. People with pacemakers or implantable defibrillators can fly without problems, since airport security scanners do not affect modern devices (carrying your device card to show security is still helpful). Mild to moderate anemia usually isn’t a problem, though severe anemia combined with the reduced cabin oxygen could cause symptoms. And despite persistent myths, having metal pins or plates from old surgeries will not cause issues during the flight itself, though they may trigger the metal detector on the ground.
The common thread is that stable, well-managed chronic conditions rarely prevent flying. What creates genuine risk is an acute, unstable, or unresolved condition where the cabin environment could make things suddenly worse, and where the enclosed, resource-limited setting of an aircraft means help comes too late.
Getting Cleared and What to Tell Your Doctor
If you have any condition on the radar, the practical step is a conversation with your doctor well before travel. Specifically, ask whether the mild drop in oxygen and the gas expansion at cabin altitude would affect your particular condition. For lung conditions, ask whether you need a hypoxia simulation test. For recent surgeries, ask how long to wait and whether the type of surgery involved any trapped gas (eye, chest, or abdominal procedures with laparoscopic gas insufflation). For cardiovascular issues, ask whether your condition is considered stable enough for the physical stress of flight.
Airlines also have their own medical clearance processes. Most carriers have a medical information form (commonly called a MEDIF or MEDA form) that your doctor fills out and submits to the airline’s medical department, which then decides whether to allow travel, with or without conditions. If you need supplemental oxygen, most airlines can arrange it, but you typically must request it at least 48 to 72 hours in advance and sometimes much earlier. Bringing your own portable oxygen concentrator is another option on many carriers, though the specific device must be FAA-approved.
Travel insurance is also worth considering if you have a condition that could flare up. Standard policies often exclude pre-existing conditions, so look for a policy that specifically covers them or that offers a “cancel for any reason” upgrade. The cost of an emergency diversion or medical evacuation from a remote destination can easily reach six figures, and no one plans to be the person who needs one.