There are a lot of things to remember while traveling, but avoiding ice in your drinks during a flight probably hasn’t been one of them. According to a study in the International Journal of Environmental Research and Public Health, if you order ice in your drink on a flight, it’s probably been prepared from tap water on the plane.

Why is This a Problem?

The study says that the supply tanks in aircraft are conducive to microbial growth, which means that anything that comes into contact with the tap water on the plane could be contaminated with bacteria.

Aircraft water tanks are rarely emptied and cleaned. According to the study, the tanks are only emptied and refilled when the plane is in for a service, if the water runs out, or during the winter to prevent the tank from freezing.

The study also showed that long-haul flights have more bacteria in the tanks than short-haul flights. According to the NBC, data from 2012 shows that during a test, one in 10 planes were found positive for carrying potentially harmful microbes in the water supply.

A lot of flight attendants also refuse to drink the tap water because of cleanliness and health concerns. One flight attendant told Business Insider that the airline workers don’t drink coffee or tea on the plane either, as they are made with tap water from its tanks.

Airlines insist the water meets regulations and is safe to drink, but the studies show otherwise. In 2004, the Environmental Protection Agency sampled water from more than 300 planes, and found that 15 percent of the samples contained the bacteria coliform. Large amounts of coliform are found in fecal matter, and the coliform count is commonly used to gauge how sanitary water is.

The EPA implemented a standard for aircraft drinking water that all airlines have had to comply with since October 2011, but there have been leaked documents from the EPA in 2013 that suggest tap water still tested positive for coliform in 12 percent of commercial airplanes in the U.S.

Despite this, airlines still say the tap water on planes is safe for drinking. Flight attendants say that although they have made a move in the right direction, it’s still not enough. Some say they only truly clean the tanks once a year, as the plane is constantly running and there is no chance to do it.

It’s probably best to err on the side of caution and drink bottled water or beverages instead.