We all know the joy of watching the airplane refreshment cart roll toward us. It’s been a long couple of hours, the cabin air feels dry, and the promise of a complimentary drink suddenly makes the rest of the flight feel a little more manageable.

But even when you’re feeling seriously parched, you might want to pause before ordering any available option. New research suggests that some beverages offered onboard may not be as safe as they seem—and here’s why.

Airline beverage
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A New Study Finds Drinking Water on Airplanes To Be “Potentially Unhealthy”
A new study examined the safety of in-flight tap water across both major and regional airlines, and the findings were concerning, to say the least. In short, “many airlines continue to provide passengers with potentially unhealthy water.”

Aircraft water systems can sit unused for long stretches between flights, experience wide temperature swings during takeoff and landing, and endure constant mechanical stress. All of these factors together can create conditions that allow unhealthy bacteria to grow.