A day after the Centers for Disease Control and Prevention urged Americans to stay home for Thanksgiving, more than 1 million people in the United States got on planes, marking the second day that more than 1 million people have flown since March. Nearly 3 million additional people have flown in the days since.

The high number of travelers speaks to a sense of pandemic fatigue that many people are experiencing. For some, the desire to see family is worth the risk of potentially getting the coronavirus while traveling.

But it’s important to remember that the current number of people flying, while increasing, pales in comparison to the number who still find the idea of getting on a plane frightening. In the 11-day period around Thanksgiving last year, a record 26 million people flew. This year, fewer than half that number are likely to travel.

How safe is flying? Numerous studies on that question have been published in the months since the pandemic brought travel to a halt in March. Many of them suggest that the risk of contracting the coronavirus while flying is very low.

Infectious disease, health care and aerospace engineering experts say that the studies — by the Defense Department, United Airlines, Delta Air Lines and others — are accurate, in part, but they all have limitations.

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One much-publicized study on flying, conducted by the Defense Department, found that “overall exposure risk from aerosolized pathogens, like coronavirus, is very low” and concluded that a person would have to be sitting next to an infectious passenger for at least 54 hours to get an infectious dose of the virus through the air. But the “54-hour” number has since been removed from the report at the request of the authors, who worried it was being misinterpreted.

Although there has been no evidence of plane flights causing many superspreader events, there have been cases of transmission. In September, a man flying from Dubai to New Zealand tested negative for the virus, but was, in fact, infected and passed it on to other passengers. The flight had 86 passengers and seven of them tested positive for the virus when they arrived in New Zealand, despite having worn masks and gloves. The seven passengers had been sitting within four rows of each other and the virus’s genetic sequence in six of seven of the positive passengers was identical.

In October, Irish officials, in a report in Eurosurveillance, a journal published by the European Centre for Disease Prevention and Control, said that 13 of 49 passengers on a 7 1/2-hour flight to Ireland tested positive for the virus, and another 46 who came in contact with the passengers in Ireland became infected.

How can you make sense of the science? What are the risk points? Here’s what we know.

What do the numbers tell us?

Or, more accurately, what don’t they tell us? We know that the coronavirus has been transported by people traveling from one place to another on planes, but we don’t know exactly how many people have contracted the virus on a plane, epidemiologists and aviation experts said. In order to know how many people caught the virus on a single flight, everyone on the flight would have to be tested as soon as they got off.

“The people who are positive as soon as they got off a plane were probably positive during their flight,” said David Freedman, an infectious disease doctor at the University of Alabama at Birmingham.

All the passengers would then need to be tested several times over a few weeks while they were isolated to ensure they didn’t get the virus after landing.

Everyone agrees airplane air is well-filtered.

Experts from various fields agree that the air on a plane cabin is filtered very well and the chances of getting the coronavirus while on a plane in flight are low. That’s because most planes have what are known as high-efficiency particulate air filters. HEPA is a designation describing filters that can trap 99.97% of particles that are at least 0.3 microns in size.

“Hospital-grade filtration occurs, and there are standards associated with that,” said Michael Popescu, a principal aerospace aircraft systems engineer, adding that the fiberglass sheets that make up the filters on planes have diameters between half a micron and 2 microns.

Air is pushed through the filter, and particles are trapped inside. Smaller particles are slowed down and kept from passing through the filter when they meet with molecules of gas, increasing the chances of their being trapped. Viruses like the coronavirus are smaller than the filters, but they tend to cluster on the larger droplets of moisture that get trapped.

Most planes recycle 25% to 30% of cabin air. The air being recycled passes through the HEPA filter, which traps virus particles. The other 70% to 75% of air is evacuated overboard every couple of minutes, meaning there is new air in the cabin every two to five minutes, depending on the size of the plane.

“The air circulation on a plane is better than in an office building, better than your apartment because the air is changed more times per hour — most planes change several times per hour, plus it’s filtered, which isn’t the case in your office or apartment,” Freedman said.

But filtration is not enough.

Ventilation is just one piece of the puzzle, said Saskia Popescu, an infection prevention epidemiologist in Arizona. (She is married to Michael Popescu.) Distancing and masking are also important to mitigate risk, and are the other key components for keeping the coronavirus from spreading, whether on planes or elsewhere.

Earlier in the year, when it first became known that social distancing could mitigate chances of getting the coronavirus, many airlines began leaving middle seats open to create more space between passengers. In recent months, however, many airlines have reversed their policies and begun seating people in all seats and saying that they are mandating mask-wearing policies, which will keep passengers safe.

Researchers said that airlines should be enforcing both social distancing policies — like leaving middle seats open — and mask wearing. Having fewer people on a plane means that there’s less of a risk of people coming into contact with someone who has the virus, said Qingyan Chen, a professor at Purdue University's School of Mechanical Engineering. “Having fewer people on the plane is key,” he said. “Fewer passengers means fewer patients, and by keeping the middle seat open airlines might remove 40% of the risk.”

A study done by scientists at the Harvard T.H. Chan School of Public Health said that “when a plane exceeds 60% load factors (percent of seats occupied), it is no longer possible to rely on physical distancing alone to mitigate the risk of virus transmission.”

Freedman and Chen emphasized that people should not be flying with homemade masks, bandannas or neck gaiters.

“For the purposes of flying, people should be wearing proper surgical/medical grade masks — the ones you can buy in boxes of 50 at a time,” Freedman said, adding that it would be wise for airlines to make it standard practice to hand out surgical masks to travelers.

And flying isn’t just sitting on a plane.

Many studies focus on the in-air cabin experience, not the parts of traveling that involve interaction with other people, often in proximity. The Harvard study focused on the three phases of air travel: boarding, cruising and deplaning.

“Each of these segments involves unique activities, such as storing and retrieving luggage, using seat trays while eating, using entertainment systems, standing in the aisle and using the lavatory,” the study’s authors wrote.

When a plane is on the ground, its air supply can come from a number of places. That air is then mixed and distributed to the cabin. One source is from the airplane auxiliary power unit, or APU, with the plane’s engine in operation. That process uses fuel and can cause noise and emissions at the airport. Air supply can also come from an airport ground source like the jet-bridge that’s known as preconditioned air, or PCA. That means air is not being circulated at the usual rate. Researchers suggest that airlines should use air from the APU for improved filtration.

“This is important since, during that time, people are exerting themselves resulting in increased respiratory levels for a brief period, raising the potential for infectious aerosols to be exhaled into the cabin,” the Harvard study notes.

Researchers also suggest that people bring smaller and fewer bags onboard, which would cut down on their exertion and reduce encounters with other travelers also putting things in the overhead bins.

Over the summer, Michael Schultz, an engineer at Dresden University’s Institute of Logistics and Aviation in Germany, and Jörg Fuchte, a senior specialist at the German aerospace company Diehl Aviation, found that the amount and type of hand luggage people brought onto the plane as carry-ons affected how long everyone was standing in line and the number of close contacts. They concluded that by reducing hand luggage, the number of close contacts encountered would be reduced by two-thirds.

The deplaning process tends to be smoother than boarding, since people naturally move in order of rows, so travelers don’t have as much to worry about. The jetway, however, can be an area of risk if too many people are allowed on without appropriate distancing, several experts said. Travelers should remain distanced from others during this process, they said, and the plane’s ventilation systems should remain on.

“The deplaning process can be enhanced by having passengers remain in their seats until directed to leave by a crew member,” the Harvard researchers suggest.

Eating and using the bathroom pose risks

Like in the cabin, air in a plane’s bathrooms is continually changed. Toilets on planes use a vacuum system to move waste to the holding tank from the toilet, so when you flush, air is pulled in through the vacuum.

“Airplane bathrooms are particularly dangerous for two reasons,” said Chen. First, he said, is the fact that you may touch surfaces that an infected passenger has just touched. “The second thing is that human waste like stool and urine contains COVID-19, and when you flush the toilet it will cause some particles to escape,” he went on. “The smaller particles carry over and could enter the air. If I have COVID-19 and use the toilet and flush and someone else comes in immediately after, that’s a risk. So far we have no evidence of people getting sick like that, but according to our models we found that this is possible.” (Over the summer this became known as “toilet plume.”)

For those reasons, experts suggest waiting 30 seconds or longer before going into a bathroom that someone else has just exited and using a tissue or paper towel so you aren’t touching surfaces like doorknobs and faucets with bare hands.

Chen also suggests that airlines stagger eating times so everyone isn’t unmasked at the same time.

“Airlines serve food to everyone at the same time, and it’s very bad because it means everyone is taking off their masks at the same time and all the particles are in the air then,” he said.

Your actions off the plane matter too.

Chen also pointed to the fact that people likely have more to worry about before getting on the plane, when they are in the terminal, going through security or sitting in airport restaurants and bars.

Others agreed. “In hospitals, people think patient interaction is highest risk, so they take a break unmasked or do charting unmasked or when they’re chatting with colleagues they take off a mask, and that’s similar to how people perceive risk in airports,” Saskia Popescu said.

“People think the plane is the riskiest, so they’ll get food and a drink at a restaurant or bar in the airport with their mask off, but that’s risky.”

This article was written by Tariro Mzezewa, a reporter for The New York Times.