Sept. 24, 2025 – With just one fatal accident for every 13.7 million boardings, flying remains a remarkably safe way to travel. But science is uncovering some hidden health risks lurking in the skies.
A jet in the air is a "very unusual environment," said Eileen McNeely, PhD, RN, an epidemiologist at the Harvard T.H. Chan School of Public Health. She and other researchers have linked some flight-related exposures – like low cabin pressure, cosmic radiation, and trace chemicals in cabin air – to higher risks of cancer, blood clots, and neurological symptoms. These risks have been most clearly documented in flight crews, who spend hundreds of hours a year at cruising altitude, but researchers say they may also affect frequent flyers. While population-wide studies support some of these associations, others remain debated and under active investigation.
As the research grows, air travel surges. The International Air Transport Association predicts that in 2025, planes will carry almost 5 billion passengers. Frequent flyers inflate the stats – a small group is responsible for as much as two-thirds of air travel. Some rack up tens of thousands of miles per year, potentially taking dozens of commercial flights.
Before you board your next flight, consider the latest science – and what steps you can take to keep yourself healthy.
Heightened Risk From Cosmic Radiation
Flying means we lose some of the atmosphere's protection against cosmic radiation, which can permeate the aircraft structure. At cruising altitudes, radiation doses are higher – and even higher on polar routes, where Earth's magnetic shield is weakest.
While individual flight doses are relatively low, they can add up. A study from the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology in Cambridge lists a one-way trip between New York and Bangkok as about 101 microsieverts, or μSv, the unit of measure for radiation exposure – so a round trip might approach 202 μSv. The International Commission on Radiological Protection recommends an annual safety limit of 1,000 μSv (1 mSv).
Radiation can cause cancer, and research shows that air crews have a higher risk of cancer. The longer they've worked, the higher the risk. A 2019 meta-analysis found that melanoma risk for pilots and flight attendants is more than double that of the general population. Female flight attendants, other studies show, have a 51% higher risk of breast cancer and four times higher risk of non-melanoma skin cancer. McNeely and colleagues published a study in May of cabin crew members showing that the body's capacity to repair DNA was significantly lower after a flight. That repair ability helps protect against cancer.
While research in frequent flyers is lacking, cabin crews are "canaries in a coal mine" for those who fly a lot, McNeely said.
If you're concerned, you can estimate the radiation dose received on specific flights using the Federal Aviation Administration's downloadable program called CARI-7 – or for a simpler online option, try the sievert calculator by the French Nuclear Safety and Radiation Protection Authority. If you approach the yearly safety limit of 1 mSv, consider adjusting your flying plans. For those who are pregnant, "avoiding long flights and circumpolar routes in the first trimester of pregnancy is the best plan," McNeely said.
Exploring Cabin Air Quality
The air we breathe on planes has also raised concerns among some researchers. Modern aircraft (with the exception of the Boeing 787 Dreamliner) use hot, high-pressure air from their engines to power some onboard systems. It's called "bleed air."
Some studies suggest that organophosphates – chemicals used in certain aviation fluids – may occasionally enter cabin air. Levels are usually "very low," said Susan Michaelis, PhD, former research head for the Global Cabin Air Quality Executive, an advocacy organization, and a former pilot. (Michaelis, a prominent figure in aviation health research, passed away on July 9 after this interview was done.) During so-called fume events, she said, higher-than-normal levels of engine oil smoke or hydraulic fluid fumes may reach the cabin due to technical problems. Research estimates of how often this happens vary widely, from as high as 1 out of 66 flights to as low as 1 in 2,000 flights.
Some researchers, such as Michaelis, have suggested a possible link between both chronic and acute exposure to certain chemicals in cabin air and a condition sometimes referred to as "aerotoxic syndrome" – a term used to describe a range of symptoms reported by some pilots and cabin crew, such as headaches, dizziness, nausea, and vomiting. "I routinely had those short-term symptoms," Michaelis said. The condition is not formally recognized by major aviation regulators, and according to the Federal Aviation Administration, "studies have shown cabin air is as good as or better than the air found in offices and homes."
As a passenger, Michaelis would carry an FFP3 mask, which she'd put on when she smelled engine fumes. (The smell reminded her of "dirty socks.") Besides putting on some form of protection, she recommended passengers speak to flight attendants if they suspect a fume event.
Changes in cabin pressure can also cause middle-ear barotrauma, a pressure-related injury of the middle ear. In one Finnish study, up to 85% of air crew members reported having at least one episode, and about 5% of them had surgery to treat it. Symptoms can range from mild to severe and often include a stuffed feeling in the ears, muffled hearing, and ear pain. In rare cases, pressure can affect the facial nerve, leading to temporary facial numbness or even paralysis (which can resemble a stroke).
Think you're having barotrauma? Some guidelines (such as from the British Thoracic Society) recommend waiting for symptoms to end before boarding a plane again; this may take between one and six weeks. Oral or nasal decongestants can help ease symptoms, according to the CDC, which also suggests that "travelers with ear, nose, and sinus infections or severe congestion might choose to postpone flying" to reduce their risk. If you have asthma or chronic obstructive pulmonary disease, it's smart to consult your doctor before travel.
How Cabin Pressure Affects Your Body
A commercial airplane's cabin is pressurized to an equivalent of 6,000 to 8,000 feet altitude. Such low air pressure reduces the oxygen passengers can breathe, potentially lowering their blood oxygen saturation levels. In one study, pulse oximeter readings of healthy travelers dropped on average from 97% to 93% at cruising altitude – a level that might warrant a call to your doctor if you were on the ground.
Hypoxia, or low oxygen levels in various tissues, forces your heart to work harder to keep the body running. When McNeely and her colleagues put 41 older adults into a low-pressure chamber to simulate a flight, they discovered that people's hearts beat faster and their heart rate variability decreased, both signs of cardiac strain. A similar 2023 study by the same team revealed that people over age 50, and those with heart or blood vessel issues, may be at risk of flight-induced arrhythmia. "That puts extra stress on the [heart]," McNeely said.
To ease the effects, pass on alcohol. In one study in a pressurized lab, drinking alcohol before dozing off led to blood oxygen plummeting to 85% on average, while heart rate rose to a median of 87 beats per minute, far higher than the 72 bpm in those who didn't drink. (Those who stayed sober with sea-level pressure averaged 63 bpm.)
One of the well-known risks of flying is venous thromboembolism, or blood clots, sometimes caused by prolonged sitting. Low oxygen on planes increases that risk. About 60,000 to 100,000 Americans die of venous thromboembolism each year, and the risk goes up by about 26% for every extra two hours you spend sitting during a flight. While the absolute risk of symptomatic venous thrombosis remains relatively low, one study found it rose from about 1 case per 1,000 people per year for non-flyers to about 3 per 1,000 for travelers on long-haul flights (four-plus hours) – roughly one extra case for every 300 people followed for a year. And "the longer you fly, the higher your risk gets," said Suzanne Cannegieter, PhD, a clinical epidemiologist at Leiden University and one of the study's authors. The study showed that for frequent flyers who took at least five flights over a period of eight weeks, the risk of a blood clot shot up about sevenfold.
Don't just blame cramped seats. In one of Cannegieter's experiments, flying activated the blood coagulation system in passengers – an effect that did not happen in a control group that sat still for the same amount of time watching movies. That's why Cannegieter recommends that air travelers keep the blood moving by frequently walking, standing up, or stretching during a flight.
This is of particular importance for those with an already higher risk of blood clots, such as women taking oral contraceptives, adults who are shorter than 5 feet, 5 inches or taller than 5 feet, 11 inches, or those who've had recent surgery. She also warned against taking sleeping pills, since these could mean "you don't breathe properly," worsening hypoxia. And while some studies show benefits of compression stockings, Cannegieter said they could offer a false sense of security, discouraging people from moving around.
For most people who fly occasionally, air travel remains safe. But, McNeely said, with the rise of ultra-long-haul routes, and with high numbers of vulnerable passengers with health issues boarding planes, we should be cautious. "I worry about all these snowbirds," she said, referring to older adults who travel to warmer climates during the winter months.