Drinking plenty of fluids on a flight is, of course, advisable - but apparently you should be sticking to bottled water only.
A flight attendant recently spoke to Business Insider about why cabin crew shy away from drinking tap water onboard.
See also: Surprising facts about germs on planes
See also: Revealed: The dirtiest part of a plane
She said: "Flight attendants will not drink hot water on the plane.
"They will not drink plain coffee, and they will not drink plain tea."
Why? Because, while they know the water tank does get cleaned, they do not know when or how often.
The Independent reports that the Association of Flight Attendants-CWA in the US said that water on board is regulated under the Environmental Protection Agency (EPA) to ensure safe drinking water on the aircraft.
However, it said: "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 paper adds that a recent study found that one in 10 commercial planes in the US had a least one positive test for coliform, and bacteria found in human faeces.
Business Insider asked a number of US-based airlines for comment and, while Delta and American Airlines reportedly declined to comment, Southwest said: "We work diligently to disinfect our aircraft water systems quarterly and monitor water quality through annual testing of all our aircraft in accordance with EPA requirements."
Don't forget that it is important to stay hydrated during a flight. It is easy to become dehydrated while flying thanks to an increase in altitude and low moisture levels.
Dehydration can worsen jetlag, dry out your skin, and give you headaches, dizziness or lightheadedness.