Flyers are often advised to drink a lot of water to stay hydrated on a flight but passengers might want to bring bottled water on board. A report shows that plane passengers are often served contaminated drinking water.

Do not drink the water warnings don't just apply to foreign countries. Travelers may be drinking contaminated water before they even get to the country as stats from the Environmental Protection Agency show that plane water is often contaminated despite federal attempts to clean up the bacteria, the Washington Times reports.

EPA launched a large campaign nine years ago to fight contaminated water, but several years later, they've found that levels haven't improved much since 2004.

According to NBC, the EPA samples water on 300 different flight in 2004 and found that about 15 percent, or one out of every 10, served water that tested positive for coliform. Finding coliform in water is a good indicator that the water may contain bacteria. The EPA found these levels to be too high and took steps to work to improve the conditions among airlines.

However NBC notes that in 2012, the levels aren't much better. Under a Freedom of Information Act request, NBC found that 12 percent of commercial flights in the US have water that continues to contain coliform which is still about one in every 10 planes.

"I would say that's still a high percentage," Bill Honker, the deputy director of the Water Quality Protection Division, EPA Region 6, in Dallas told the Washington Times. "There is more that needs to be done," he acknowledged.