They may not tell you, but there are a few things flight attendants would never drink in the air.

Overnight and early morning flights are hard on all of us. Sometimes, the only saving grace is a nice warm cup of tea or coffee to wake us up and help us feel refreshed after hours in the air. But you may want to think twice before you order that English breakfast blend, according to cabin crew members.

“Flight attendants will not drink hot water on the plane. They will not drink plain coffee, and they will not drink plain tea,” one flight attendant told Business Insider in February. Why the self-imposed ban on delicious warm beverages?

As NBC 5 noted, the water for tea and coffee comes from the tap, not from a bottle, while in flight. And that water could be downright disgusting. According to a 2004 EPA sample of 158 planes, 13 percent contained coliform. Two of the airplanes were found to have dangerous E.coli in the water. And as Business Insider reported, an additional EPA study found that one in every eight planes fails the agency’s standards for water safety.

"Water onboard is regulated under the Environmental Protection Agency to ensure safe drinking water on the aircraft,” the The Association of Flight Attendants-CWA, told Business Insider. “The Association of Flight Attendants-CWA pushed for this regulation over 15 years ago. The regulation gives broad discretion to airlines on how often they must test the water and flush the tanks. AFA does not believe this regulation goes far enough or is sufficiently enforced."

The bacteria is likely delivered while the water is in transit, NBC reported. According to a 2015 study in the International Journal of Environmental Research and Public Health, more microorganisms exist in the transport vehicles than in the water’s original source, thus transferring the organisms from truck to plane.

Dr. Cedric Spak, an infectious disease specialist at Baylor University Medical Center, told NBC that people with a compromised immune system should steer clear of the in-flight beverage. He additionally noted that people with infants should “think twice” before filling a baby’s bottle with airplane tap water.

“That doesn’t sound like a very good idea,” he said.