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(CNN) - You have probably never thought much about the quality of water in an airplane.
LOOK: What are the rules about alcohol consumption for US airline pilots?
Maybe it's time for you to do it.
A recent study classifies water served in major regional and US airlines.
Drink if you are on an Alaska Airlines or Allegiant Air flight. Those two airlines tied for first in the study's ranking, from the Hunter College New York City Food Policy Center at New York City University and the DietDetective.com website.
But you may have to be careful if you are on a JetBlue or Spirit flight. They tied last among the major airlines in the study, which left at the end of August.
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CNN contacted Spirit and JetBlue to comment on their low rankings and is waiting to receive a response.
The study was not a review of bottled water that is often served on airplanes, but a look at the water used to make items such as tea and coffee, as well as the water used in the bathrooms.
What should you do?
A list of recommendations for air travelers in the study includes warnings not to drink water on board an airplane unless it is in a sealed bottle or drink coffee or tea prepared on board.
And instead of washing their hands in the plane's bathroom, the study encourages passengers to use hand sanitizers.
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Airlines must provide safe drinking water for passengers and crew due to the Airplane Drinking Water Standard, a federal government policy dating back to 2011.
The airlines obtained “water health scores” ranging from 5 (the highest score) to 0 (the lowest).
"A score of 3.0 or better indicates that the airline has clean and relatively safe water," says the study.
Criteria for the study included the size of the fleet, the number of violations of the Drinking Water Standard, the results of the water sample reports and how cooperative the airlines were to provide answers to questions about water quality.
Here are the rankings
The main airlines, and their classifications are:
Alaska Airlines: 3.3
Allegiant Ai:, 3,3
Hawaiian Airlines: 3.1
Frontier Airlines: 2.6
Southwest Airlines: 2.4
Delta Air Lines: 1.6
American Airlines: 1.5
United Airlines: 1.2
Spirit Airlines: 1