Staying hydrated on a plane is important, which typically means staying away from alcohol and caffeine. While you might be tempted to fill your coffee cup as the flight attendant is walking around with the coffee pot, there are a few reasons you might want to reconsider. Here's why you should avoid drinking coffee — or hot tea — on your next flight.

Water Comes From the Tap

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Water that is used for making coffee and tea on a plane comes from the tap. You might be wondering what the big deal is since it’s boiled anyway. Unfortunately, the conditions of where the water comes in from can be downright disgusting. Flight attendants don’t even know how long it’s been since the last time the water tanks were cleaned.

Studies and Reports on Airplane Water Show Bacteria Present

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There have been numerous studies and tests done on airplane water contamination, and the results are a tad disturbing. The International Journal of Environmental Research and Public Health published a paper several years ago about the quality of aircraft water. They identified 37 different types of bacteria in the onboard water supply. Most were bacteria naturally found in water, but two were not. In this case, the paper said that the bacteria didn’t fall into the dangerous categories. However, some travelers are still at risk of becoming sick, especially those who have compromised immune systems.

Years ago, the EPA launched an initiative to improve the safety of onboard water supplies after tests done in 2004 revealed bacteria. In the initial study, they sampled around 300 planes and 15% tested positive for coliform, which is found in human feces. While coliform might not make someone ill, it suggests the presence of other more harmful bacteria like E. coli.

The EPA requires that airlines test for coliform and E. coli at least once a year. If the tests are positive, they have to flush the tanks and retest again. Part of the issue is that water tanks are filled at airports all around the world. The hoses can be dirty and introduce bacteria. Even more microorganisms can be in the transport vehicle transferring them to the plane.

Some Flight Attendants Refuse to Drink Hot Beverages

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In a Vice article, an anonymous flight attendant warned the public not to drink the coffee on the plane. The unnamed flight attendant went on to say that their plane was recently tested for E. coli and they did not pass. However, maintenance came on and, after hitting a few buttons, they suddenly passed. That doesn’t instill a lot of confidence that the water is actually clean, clearly. Whether this is an isolated incident, a disgruntled employee fabricating a story or, God forbid, the norm, we can't say.

Jay Robert, creator of a Fly Guy’s Cabin Crew Lounge, told The Points Guy that he’s spoken with crew who’ve ended up becoming very sick after consuming hot beverages on a plane. On the flip side, some flight attendants aren’t phased by the reports. Some drink coffee and have never been sick, and the same goes for passengers who’ve traveled hundreds of thousands of miles without intestinal issues after drinking coffee on all their flights.

Should You Drink Water on the Plane?

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If you’re worried about water that is served with the drink service, you don’t need to be. This is bottled water, so no reason to refuse it. But should you order ice with that water? Some people suggest staying away from the ice cubes as well. There have been reports that say ice comes from the same contaminated water source, the trays are rarely cleaned and studies show bacteria present here too.

So, what should you do if you can’t go without a cup of coffee on your flight? Consider purchasing your coffee at the airport and bring it with you or drink it before your flight departs.