What we know about whether and when to mask on planes
Story at a glance
The mask mandate for air travel has been dropped.
Air ventilation and filtration systems on planes are effective when they are turned on, which is not the entire time passengers are on board.
Without a mandate, passengers must think about their personal risk and decide if they want to wear a mask.
Recent changes in mask mandates have renewed the debate on whether masks are needed on planes and other forms of transportation.
Studies suggest that transmission of the coronavirus on a flight might be low, but the risk is still there. Ventilation and filtration systems are effective when turned on, but air circulation is variable in airports and on planes before and after a flight.
What we generally know
Air filtration systems on planes are effective at filtering out pathogens in the air, including the SARS-CoV-2 coronavirus. A report from the Department of Defense states that plane ventilation and filtration reduce the risk of airborne exposure by 99 percent. According to The International Air Transport Association, cabin air may be refreshed 20 to 30 times an hour.
However, the key thing to note about ventilation and filtration systems on planes is that they are not turned on 100 percent of the time. They are typically powered by the main engines and are off during boarding and taxiing unless they are powered by other means.
Once turned on, they take time to cycle out and filter the old air and push in new air. And as epidemiologist Katelyn Jetelina points out, the contaminated air needs to make it to the filter in the first place to protect someone from inhaling it, and filtration isn’t effective for larger droplets.
Overall, if you are sitting near someone who is infected, then your risk of catching the coronavirus is higher. In a study of a 10-hour flight from London to Vietnam in March 2020 with no mask mandate, a person in business class had the virus. That led to 12 other people in the same section of the plane and four people elsewhere on the plane getting infected.
If there is an infectious person on a flight, the people who are two rows in front and behind seem to be at most risk. That said, people will move around on a plane so it is still important to consider the risks whether you are the one moving around or others on your flight are the ones taking a lap to stretch their legs.
What we don’t fully know
While ventilation and filtration on a plane may be reliable, other sections of the journey may not be. This includes time spent in the airport, at the gate, on the jetway and moving through the aisles on the way to a seat.
To illustrate the differences, aerosol scientist Jose-Luis Jimenez documented the carbon dioxide levels as he travelled internationally in a Twitter thread. The amount of carbon dioxide is a rough indication of air filtration, as people will convert oxygen to carbon dioxide when they are breathing. Carbon dioxide will slowly build up in areas that are crowded with a lot of people and/or if air circulation is poor.
By comparing the relative numbers at different points in his journey, Jimenez saw that carbon dioxide levels rose rapidly when he was on the plane and boarding was still ongoing. Although those numbers can give us an impression of how well each space is ventilated, it’s only a rough approximation with many assumptions that go with it. It depends on the respiration rate of the people in a space, how crowded a space is and how well the area is ventilated.
Most crucial times to wear a mask
Generally, we know that wearing a mask can protect people from infection. A review of studies found that masks are effective at reducing transmission on planes, though the type and fit of the mask also make a difference.
If there is no mask mandate, when to wear a mask during a journey is a personal decision. If someone symptomatic is nearby, then it may be in your best interest to don a mask. If you are moving around, you are increasing the circle of people you are exposing yourself to and that could increase your risk.
Being in areas that are obviously not well-ventilated such as jetways on the way to board a plane is riskier than being in a part of an airport that is well-ventilated.
When on the plane, it may be best to wear a mask while people are boarding, and when the plane is taking off. It may be less risky to take off your mask when the plane is in the air and ventilation and filtration systems are on and have run for a while. It may, however, also be more risky to be maskless while everyone is eating and drinking.
Without a masking policy, each passenger is left to decide whether and when they choose to mask and what amount of risk they are willing to take on. Deciding to take a flight is the first point of accepting risk, and every stage before and after the flight presents its own collection of decisions.
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