As much as airlines make an effort to keep their planes clean, large fleets and short turnaround times simply don't allow them to sanitize every surface.
Which leads us to the obvious question: What are the dirtiest places on the plane?
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According to a study conducted by microbiologists hired by Travel math, seat-back tray tables are a hotspot for bacteria.
In the study, microbiologists found an average of 2,155 colony-forming units 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.
But Drexel Medicine, the healthcare system affiliated with Philadelphia's Drexel University College of Medicine, has its own list.
Here are three other parts of the plane they found that are just as germ-filled as the tray tables.
The first area shouldn't surprise anyone — airplane bathrooms or lavatories.
"With roughly one restroom per 50 passengers, they're one of the germiest places on a plane and a breeding ground for bacteria like E. coli," Drexel Medicine wrote on its website.
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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 a recent 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.
Bringing your own reading material is the less germy and more advisable alternative to in-flight magazines.