In March 1977, on a Boeing 737 making a run from Anchorage to Kodiak, a bunch of people got the flu.
That’s not supposed to happen on airplanes. Influenza is a respiratory virus, most likely transmitted at least in part via airborne particles, and airplanes have recirculation, air filtration, and fresh-air injection systems burly enough to purge cabins of smoke if there’s a fire. They ought to do the same with a virus. So how’d those people get sick? That’s the interesting part.
The flight made a stop on the way, in Homer, where a new passenger came on board. That person, the “index case,” had the flu. The 737 took off, but an engine problem forced it to return to the airport. There it waited—engines off, on the tarmac. Some passengers stayed on board; some waited in the terminal. Eventually, other planes took the stranded passengers on to their destinations, where nearly three-quarters eventually became sick with the flu as well—“secondary infections.” Almost all of them were the ones who’d spent their unplanned layover on the plane with the sick passenger rather than in the terminal.
That choice—where they waited—was the crux. With the engines off, the 737’s air conditioner wasn’t working. The airplane cabin turned into the classic indoor, crowded, low-ventilation space you really want to avoid if you don’t want to pick up a respiratory virus like the flu—or, more relevantly today, Covid-19.
It might not seem like it, but this story is actually good evidence for why it’s hard to get sick on an airplane, although folk wisdom might suggest otherwise. If everyone’s wearing a mask and the ventilation is blowing, it is indeed difficult to get Covid-19 when you fly.
But it’s not impossible. And that’s where things get complicated.
As most of the United States tips back into uncontrolled spread of the pandemic virus, with just a week before the traditional holiday travel season begins, it’d be nice to know: How risky are airplanes, exactly? The short answer is: No one knows, but risky enough. If you can avoid flying, avoid it. “The position that a lot of airline executives are stating now, publicly—that the risk is essentially zero—is just unrealistic,” says David Freedman, an infectious disease researcher at the University of Alabama at Birmingham who studies travel medicine. “I’m not saying the risk is high. And the risk is manageable. But to start from the position that the risk is zero on an airplane is like the politicians who are saying this is a hoax. It exists.”
This position isn’t perhaps the sexiest. “Low-ish risk but nonzero, avoid unless you can’t” bears some explaining.
Most scientists and health care workers now agree that the virus that causes Covid-19 can travel through the air, borne aloft in teeny globs of snot. Even people without symptoms can emit these particles, just by talking or breathing. Masks can stop some of them, though probably not all—but hey, nobody really knows how many you have to inhale to get sick, either. Another thing that helps prevent infection is being far enough away from people that whatever virus they’re giving off gets diluted before it gets to you; even better is being outside.
Airplanes are definitely inside, and a fully-booked coach cabin doesn’t allow much leeway for social distance. But actually, the filtration and ventilation systems on airplanes are pretty great. Jet engines inhale great gulps of air, mix it with fuel, and set it on fire, which makes thrust. But some of that air (before the fuel and fire parts) gets diverted into the ventilation system. Because it comes from outside, at cruising altitude, that air is very clean and very cold. It feeds through air compressors (which heat the air way up, to hundreds of degrees) and then into air conditioner “packs” in the fuselage which cool it back down and send it to a manifold that mixes it with air from inside the cabin. It goes through ducts in the ceiling to vents that blow it downward through the cabin, and then it gets sucked into intakes next to passengers’ feet. About half of that then gets sent back outside, and the other half gets run through HEPA filters—very good at getting rid of even viral particles—and back to the manifolds, to meet a new gust of outside air. In a plane where everything’s working up to spec, the whole cabin gets an air change every two to three minutes. Tobacco smells, dust, pollen, germs, halitosis, sweat, farts—all gone. Whoosh.