The quality of airline coffee is less about the coffee beans and more about the water. The water used for your in-flight coffee comes from a tank that gets filled upon landing.

These water storage tanks are not regularly cleaned, which could result in bacterial growth. A 2019 study on select American airlines deemed onboard water as potentially unsafe.

A flight attendant also took to TikTok to describe how their plane’s coffee makers are never cleaned, while the pots are emptied between flights — and only cleaned with hot water.

Meanwhile, the larger jugs used to hold coffee during the day are cleaned once per day, after multiple flights. To ensure safety, grab a coffee before take-off or after landing.