Flight attendant serving coffee aboard an American Airlines flight

Alaska and Allegiant airlines had the best onboard drinking water while JetBlue and Spirit had the worst, according to a new study.

The 2019 Airline Water Study was a joint effort by DietDetective.com and the Hunter College NYC Food Policy Center at the City University of New York. The report reveals that the quality of drinking water varies by airline, and many airlines have possibly provided passengers with unhealthy water.

The 2019 Airline Water Study ranked 11 major and 12 regional airlines, mainly by the quality of water they provided onboard its flights. Each airline was given a “Water Health Score”—5 being the highest, 0 being the worst—based on 10 criteria including fleet size, ADWR violations, positive E. coli and coliform water sample reports and cooperation in providing answers to water-quality questions.

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A score of 3.0 or better indicates that the airline has relatively safe, clean water.

Here’s how the major airlines fared:

—Alaska Airlines, 3.3

—Allegiant Air, 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

—JetBlue, 1

—Spirit Airlines, 1

Unhealthy water violates the federal government’s Aircraft Drinking Water Rule (ADWR), which was implemented in 2011 and requires airlines to provide passengers and flight crew with safe drinking water.

The 2019 Airline Water Study also finds that the Environmental Protection Agency—one of the federal agencies responsible for ensuring safe aircraft drinking water—rarely levies civil penalties to airlines in violation of the ADWR.

To be safe, DietDetective.com and Hunter College suggested never drinking any water onboard that isn’t in a sealed bottle; do not drink coffee or tea onboard; do not wash your hands in the bathroom.