Hunter College NYC Food Policy Center at the City University of New York conducted a study on airline water quality, finding many airlines have potentially provided passengers with dangerous drinking water.

While the Aircraft Drinking Water Rule, put into place by the federal government in 2011, requires airlines to test water samples from water tanks regularly, the federal government, according to the study, rarely penalizes airlines for unhealthy water onboard aircraft.

“Alaska Airlines and Allegiant win the top spot with the safest water in the sky, and Hawaiian Airlines finishes No. 2,” said Charles Platkin, executive director, Hunter College NYC Food Policy Center. “Except for Piedmont Airlines, regional airlines need to improve their onboard water safety.”

Piedmont Airlines, which operates American Eagle flights, was the highest-rated regional carrier. Among major airlines, Spirit and JetBlue tested the worst for on-board water quality. Republic Airways, which flies for United Express, Delta Connection and American Eagle, and ExpressJet showed the worst test results among regional carriers.

Unsafe water aboard aircraft may include coliform bacteria and E. coli. The Hunter College NYC Food Policy Center advises travelers to never drink water onboard an aircraft unless it is sealed in a water bottle, to never drink coffee or tea while flying and to use hand sanitizer in the aircraft restroom.