[SciShow intro plays]

Olivia: Have you ever gone on long flight somewhere, then gotten sick within a day or two of landing? The way you got sick might seem obvious: the airplane, with all that recycled air making you breathe in everybody else’s germs.

But even though planes can expose you to disease, they aren’t, like, giant germ incubators. Let's start with the idea that planes are recycling their air, picking up germs from people 10 rows behind you and blasting it in your face through the vents. That’s only partially true.

Sure, planes do use recycled air. About half of the air on most planes is filtered and recirculated over and over again. The rest is fresh air that comes in from outside the plane through the engine compressors. It’s mixed in with the recirculated air in the cabin. Just because air is recirculated, though, doesn't mean it's going to be a hotbed of contagious disease.

In fact, most planes use high-efficiency particulate air, or HEPA, filters, the type that hospitals use in their operating rooms and intensive care units to keep the air free of bacteria and viruses. These filters work by letting air flow through tiny holes that other particles get blocked in, kind of like a colander for dust and bacteria. To be certified as a HEPA filter, a filter has to be tested with particles that are just 0.3 microns — that’s 3 ten-thousandths of a millimeter — and it has to block at least 99.97% of them.

The air on a plane is recirculated 20-30 times an hour, giving it plenty of chances to be filtered. What's more, the air that comes out of the vent above your head leaves the cabin through a grill along the wall in or very near your row. This means the air isn't flowing forward or backward through the plane. So the air you're breathing probably isn’t carrying bacteria and viruses from people who aren't already sitting right next to you. Of course, if the person in the seat next to yours sneezes all over you, then you do run the risk of catching something.

But that has less to do with the air on a plane and more to do with being close to other people, just as you might be on a bus or train, or at school or your office. So if you get sick on a plane, don't blame the air. Blame your seat assignment.

Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming. If you’d like to submit questions to be answered, or get some videos a few days early, go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!