Why you get sick when you fly

American Airlines A321–231 departing JFK runway 31L with a disappointing number of closed window shades

“I’m sure that cough is nothing.”

As aircraft ascend to cruising altitude, the outside air pressure drops dramatically. The now relatively high air pressure in the cabin pushes outward on the aircraft’s structure with greater and greater force. To avoid dangerous levels of stress, the cabin’s pressure is also lowered, up to an equivalent altitude of 2400 meters (8000 feet). That’s why your ears pop — gas expansion causes air to escape the middle ear and sinuses as cabin pressure drops. And why there’s that teeny tiny hole in the window to let the air pressure equalize between the window dust cover and the structural window panes.

Because of the relationship between pressure and temperature, the outside temperature has also dropped to between -45° and -55° Celsius. At this altitude, air’s capacity to hold water vapor is about 1/1000th what it is at sea level. And the cabin is fed with this outside air. That dull headache and fatigue you’re beginning to feel is dehydration. While that sucks, the real cause of what makes you sick is what this dry air does to your mucociliary clearance system.

Without humidity, a thin film of mucus in your respiratory system — which acts as a filter carrying away bacteria and viruses to protect you from getting sick — shuts down. While you’re buckled helplessly in your seat, the microorganisms breathed out by hundreds of people are scouting out real estate inside you with every breath you take. If your mucociliary clearance system is an expert swordsman, the lower-than-Death Valley humidity level is Indiana Jones with a pistol.

“Why don’t they add some fucking humidity?”

That’s probably the question running through your mind.

Well, this low humidity is definitely in your best interest. Cabin air has a surprising source of humidity— the vast majority of water vapor in an aircraft cabin comes from passengers exhaling. A long-haul flight can easily accumulate hundreds of liters of condensation which can damage electrical systems, freeze rudders, change the weight distribution, and corrode metals to the point of structural failure. If even a tiny hole starts leaking air, the immense pressure differential could lead to explosive decompression, which I think we can all agree is worse than getting sick.

There’s a glimmer of hope on the horizon; composite materials like carbon fiber reinforced plastic are replacing metals in newly designed aircraft. Composites are used to reduce weight because they are stronger than metal of the same weight. Lighter weight reduces fuel consumption and therefore cost. The happy side-effect is composites are strong enough to safely tolerate higher, more comfortable cabin pressures and humidity levels because they don’t corrode.

So for now, here’s what you can do:

1. Stay hydrated—drink a liter of water every six hours—and avoid things that dehydrate you like protein, alcohol, and anything salty or sugary (caffeine doesn’t dehydrate, as often believed)
2. Wash your hands and avoid touching your face
3. If you absolutely must avoid getting sick wearing a surgical mask will do the filtering your body can’t
4. High-five your scientist and engineer friends