The Real Reason Why You Get Sick After Long Flights

For many, coming down with a cold after a long flight is all but inevitable.

But why is that? What do we get sick after taking a long flight?

The easy answer is that there are a couple of hundred people trapped in close proximity to one another inside a pressurized metal tube for hours on end, making for a rich breeding ground for germs.

However, the reality is that things are a bit more complex than that.

Contrary to popular belief, airplane cabin air isn’t as dirty as you think.

According to the International Air Transport Association, the risk of getting sick from flying is similar to that of other high-density activities like going to the movies or taking the train. IATA claims that in-cabin HEPA filters can get rid of 99.9995% of germs and microbes in the air. Plus, cabin air is only half recirculated air. The other half is fresh air pumped in from the outside.

But that doesn’t mean the cabin environment can’t contribute to you getting sick.

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If someone is sitting next to you has a cold, then all bets are off. And then there are the many germy surfaces on board an aircraft.

According to a study conducted by microbiologists hired by Travelmath, seat-back tray tables are a hotspot for bacteria. In the study, microbiologists found an average of 2,155 colony-forming units (CFU) per square inch on tray tables collected from four different planes.

In contrast, the study found an average of “just” 265 CFU a square inch on the lavatory flush button, which itself is far from clean

Drexel Medicine, the healthcare system affiliated with Philadelphia’s Drexel University College of Medicine, called airplane bathrooms or lavatories, “One of the germiest places on a plane and a breeding ground for bacteria like E. coli,” Drexel Medicine wrote on its website.” In fact, the healthcare professionals advise against flyers directly touching anything in the lavatories with their hands. Instead, they suggest the use of paper towels when touching the faucet or toilet seat lid.

According to Drexel, another area to avoid touching on planes are the seatback pockets.”From used tissues to fingernail clippings and dirty diapers, people stuff all kinds of germ-infested materials into airplane seat pockets,” Drexel Medicine wrote.

That conclusion is backed up by an Auburn University study that found that bacteria can survive in seatback pockets for up to a week.

This leads to the next area to avoid — in-flight magazines.”While it may be tempting to pick up that issue of SkyMall, think about how many people have thumbed through those pages,” Drexel wrote. The website reminds readers that the magazines are really only “cleaned” once every quarter when they are replaced.

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It’s not the flight, it’s you

However, germs aside there are other reasons why travelers are susceptible to illness after a long flight. One of the benefits of long-haul flying is that it can take you halfway around the world in a matter of hours. Unfortunately, it takes our bodies much longer to adjust to our new environments. Our inability to adjust results in a host of symptoms we refer to as jet lag.

“The fundamental basis of jet lag is the disruption of your body clock system,” University of Sydney Professor Steve Simpson told Business Insider. “We have what’s known as a circadian clock system that organizes everything about us.”

“It’s a very sophisticated clock system which resides in every cell and organ in our bodies and is controlled by a master control clock in our brain.”

Each cycle of the circadian clock runs about 24 hours and is reset each day by a series of cues such as light, temperature cycles, and food.

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According to Prof. Simpson, who is the academic director of the University’s interdisciplinary Charles Perkins Centre that’s dedicated to improving global health, our circadian clock regulates a slew of body functions including sleep, alertness, activity periods, metabolic cycles, and digestion.

A recent Cambridge University study has also shown that a disruption to a person’s circadian rhythm can compromise their immune system, making people with disrupted circadian rhythms more susceptible to infections.

Since your internal body clock can adjust no more than an hour to an hour and a half every day, passengers on long-haul international flights will endure days of circadian rhythm disruption on every trip.

As a result, Prof. Simpson recommends passenger traveling on long flights begin shifting their circadian rhythm ahead of any planned travel. A few days before your trip, gradually shift your eating, sleeping, and activity patterns along with your light exposure to match that of your destination, Simpson said.

This article originally appeared on BusinessInsider.com