Joseph G. Allen is an associate professor and director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health. He co-wrote “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.”
In fact, it’s time for the Transportation Security Administration to take a step back and review its mask mandate for flights, which expires on April 18. The science supports relaxing the rule during flights — provided other key measures are in place to protect passengers.
As I’ve written previously, people are no more likely to contract diseases on a plane than in any other setting, contrary to conventional wisdom. I’ve studied airplane air quality for over a decade, and served as one of the lead authors of a report for the Transportation Research Board of the National Academies on infectious-disease transmission in airports and on airplanes, so it’s a topic I know well. Even at the beginning of the pandemic, the science was clear that although transmission can happen on an airplane, it’s very rare. The scientific evidence accumulated since the start of the pandemic confirms this.
People do get sick when they travel, but the airplane is one small part of the trip. There are Uber and subway rides, security lines, staying with friends and family, going out to restaurants. Not to mention bad sleep and maybe changing time zones. All of this can contribute to the risk of getting sick. In reality, one of the safest parts of an entire trip is when travelers are seated in the airplane and the systems are running.
Why? Airplanes have what are called environmental-control systems that determine the craft’s ventilation rate. About 50 percent of the air you breathe in an airplane comes from outside, so it’s virus-free. That air is called “bleed air” because it comes from the engine, where a small portion is “bled” from the engine into the cabin. It’s conditioned before it reaches you.
The other 50 percent of the air is recirculated. All of that air passes through HEPA filters, which provide a minimum capture efficiency of 99.97 percent. That’s better than any filter in any building you’ve ever been in, with the exception of some parts of a hospital.
As for ventilation, airplanes get more than 10 air changes per hour. Again, about the only place with as high a ventilation rate is in some, but not all, hospital settings, such as surgical suites.
There is something else that helps keep airplanes safe: ventilation effectiveness. Not only is that air being pumped into the cabin at high rates, but it’s also being delivered at every row, and exhausted at every row, down by your feet.
Why does this matter? Well, consider an old building, which might have a vent in one corner of the room and exhaust on the other side. There can be “dead zones” where the air doesn’t fully mix. This doesn’t happen on airplanes, because air is delivered to each person, at their seat.
But there is one part of the airplane experience that can be higher risk and that I have been warned about since that 2013 report: boarding and disembarking. When planes are at the gate, their ventilation systems aren’t always running. If they’re not running, there is no air coming into the cabin, and, importantly, no air running across those great HEPA filters. Combine that with high-occupant density and a small volume space, and you have a recipe for transmission. In fact, the one high-profile airplane outbreak of covid from a long-haul flight involved a plane that stopped for refueling and shut off the air supply while on the tarmac.
Airplanes can address this by connecting to ventilation systems at the gate or by running their auxiliary power units. The problem is that it doesn’t seem to be happening. On my last several flights, I measured carbon dioxide levels during boarding and found that they were at greater than 2,500 parts per million (compared with the 1,000 ppm or so that you’d find in a typical office). That’s a clear indication that the ventilation system was not running. Others have posted similar results on Twitter.
So, here’s what should be done. First, airlines need to follow the guidance here that has been around for more than a decade and make sure ventilation is running while at the gate. Second, airlines should require masks during boarding and disembarking, when there is greater mixing of people in the cabin and when people are exerting themselves more and therefore emitting more respiratory particles.
Once the systems are running, the masking rules during flight can be relaxed. Importantly, anyone who feels this places them at high risk should get vaccinated and boosted and wear an N95 mask. This, in addition to the good ventilation and filtration, will provide excellent protection, regardless of whether others on the plane are wearing masks.