A new study came to a surprising conclusion about the one thing you should never consume on a commercial airliner.

No, it's not alcohol. Their counterintuitive advice? Never drink tap water, coffee or tea on a commercial airplane. It's counterintuitive simply because dehydration is a big issue for many airline passengers.

But, the study authors, at Hunter College NYC Food Policy Center and DietDetective.com, say their concern is that the tanks in which airline water is stored be contaminated with harmful bacteria.

Their advice, verbatim:

It's probably best to avoid drinking water from the tap on a plane, which also means staying away from coffee and tea. Even though the EPA has instituted the Aircraft Drinking Water Rule (ADWR) to "ensure that safe and reliable drinking water is provided to aircraft passengers and crew," there are a couple of reliable researchers who believe there may be harmful bacteria in airline water. 

This is probably because the water tanks are not emptied very often (you would think they'd be emptied and cleaned at least once a day, but this is not so). So water is just sitting for long periods of time in what appear to be not-so-clean tanks.  

Instead, they advise buying bottled water in the airport, and if you want coffee or tea, asking the flight attendants to heat it up for you.

Now, we should mention that just about every airline takes issue with this recommendation, and with the suggestion that they don't clean water tanks often enough. But they're mixed on whether they claim to do so as often as the study suggests should be the norm (versus the EPA regulation). 

Among the responses included in the report:

  • Delta Air Lines: "In compliance with regulation, the team disinfects the tank system on every aircraft four times a year, performs water quality sampling once per year, and enters this data into EPA's Aircraft Reporting and Compliance System."
  • JetBlue: "JetBlue is in full compliance of all requirements outlined in the EPA's Aircraft Drinking Water Rule (ADWR)."
  • Air Canada: "Air Canada samples every aircraft 4 times per year, in addition to the random testing conducted monthly by Health Canada."
  • American Airlines: "Our top priority is to provide our customers with a safe and pleasant travel experience. All food and beverages on board our flights must meet industry - and where applicable - regulatory standards."
  • United Airlines: "We utilize a vigorous program that complies with Federal EPA regulations. In addition to the disinfection program, United monitors water quality by testing samples at EPA-approved laboratories - every single plane in the fleet is tested annually."
  • Hawaiian Airlines: "At Hawaiian food safety is a top priority at all of our kitchens and we have an audit program to regularly review the food safety records at each of our vendors."
  • Southwest Airlines: "The regulations include requirements for regular testing of aircraft water, as well as regular disinfection of each aircraft's water system. Southwest Airlines meets or exceeds these regulations, and we are proud of our compliance record."

You can see each airline's longer response in greater detail at Diet Detective. But at the same time, this isn't the first study to make the exact same recommendation. And anecdotally, some airline employees say they'll never drink the water on board.

So I guess, instead of buyer beware--drinker beware.

Separately, the study also examined the best and worst food choices on 11 different U.S. airlines. Here they found some good news: The average number of calories per airline meal dropped a bit, from 405 to 373 calories.

Winners in the category included Alaska Airlines, Delta Air Lines, Air Canada and JetBlue. 

Bottom line: Nobody's flying economy because they're excited about the food or a weak cup of coffee.

Maybe you're better off just bringing your own food and drinks aboard. If you're not really worried for health reasons, at least you can enjoy a meal or a drink on your schedule, not your flight attendant's.