The 2019 Airline Water Study from Hunter College NYC Food Policy Center and DietDetective.com ranks the safety of the onboard water of a dozen major airlines. only 3 are safe, the rest? Bring your own water, don't even drink the coffee onboard

“Alaska Airlines and Allegiant win the top spot with the safest water in the sky, and Hawaiian Airlines finishes No. 2,” says Charles Platkin, PhD, JD, MPH, and executive director of the Hunter College NYC Food Policy Center.

Here are the full rankings of airlines’ onboard drinking water, with their scores (zero is the lowest, five is the highest). According to the study, “a score of 3.0 or better indicates that the airline has relatively safe, clean water.” 1. Alaska Airlines: 3.3
2. Allegiant Air: 3.3
3. Hawaiian Airlines: 3.1
4. Frontier Airlines: 2.6
5. Southwest Airlines: 2.4
6. Delta Air Lines: 1.6
7. American Airlines: 1.5
8. United Airlines: 1.2
9. JetBlue: 1
10. Spirit Airlines: 1

For travelers concerned about onboard water quality, the study has three suggestions: “Never drink any water onboard that isn’t in a sealed bottle, do not drink coffee or tea onboard, and do not wash your hands in the bathroom; bring hand-sanitizer with you instead.”

https://www.usatoday.com/story/travel/airline-news/2019/09/17/us-airlines-cleanest-and-dirtiest-drinking-water/2347159001/
https://www.eurekalert.org/pub_releases/2019-08/tcuo-2aw082919.php