While air travel is convenient, there's never any promise it will be particularly clean. Security checkpoint bins and airplane tray tables are notorious for harboring germs. In addition to being careful of what you touch, you need to be cautious about what you drink.

Air travelers have been warned in the past about the questionable water quality of major airlines. Owing to inconsistent water transport issues, storage methods, and lackadaisical monitoring, several studies and investigations—including a 2004 report by the U.S. Environmental Protection Agency (EPA)—have found bacterial contamination and even insect eggs lurking in liquid served to passengers in the form of coffee and tea.

Unfortunately, the problem doesn’t appear to be getting any better. A new study on the quality of airline tap water conducted by the Hunter College NYC Food Policy Center found E. coli, coliform bacteria, and other unpleasantries across 11 commercial and 12 regional carriers.

The study ranked airlines according to water quality tests, and whether airlines were forthcoming in disclosing how they handle water transportation and storage for in-flight plumbing, using a scale of 1 to 5. While some airlines, like Alaska and Allegiant, scored well at 3.3, others (including JetBlue, Spirit, Delta, and United) ranked poorly in terms of delivering healthy, clean water, which is often sourced from local municipalities.

Low scores also indicated a lack of transparency about the airlines' water monitoring process. Moving the water from its source through hoses and tanks can create opportunities for the water to become contaminated.

How can questionable water be served? While the EPA introduced an Aircraft Drinking Water Rule in 2011 that mandated quarterly cleanings of airplane holding tanks and bacteria tests, Condé Nast Traveler reports that the agency does little to enforce it, typically opting not to issue fines to airlines found to be in violation of the terms.

The study’s authors recommend people avoid drinking tap water, coffee, or tea while on airplanes and should instead opt for bottled water. Because the stored water is also used for lavatory sinks, it’s possible you might introduce germs to your hands even after “cleaning” them. Hand sanitizer is recommended.

[h/t Condé Nast Traveler]