Some of the country’s biggest airlines are serving passengers tap water that may be unsafe to drink, according to a new three year analysis of onboard water systems in the United States.

The study, released in early January 2026 by the Center for Food as Medicine and Longevity, found widespread contamination issues on board many aircraft and singled out one major U.S. carrier, American Airlines, as the worst performer among large airlines for in flight water quality.

Researchers are urging travelers to avoid any water on planes that does not come from a sealed bottle, including the water used for coffee, tea and ice.

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A sweeping new look at water quality in the skies

The 2026 Airline Water Study examined the tap water that flows through aircraft galleys and lavatories, not the bottled brands handed out during beverage service.

Researchers compiled and analyzed 35,674 water sample locations collected between October 1, 2022, and September 30, 2025, across 10 major and 11 regional U.S. airlines. Each carrier received a Water Safety Score on a scale from 0 to 5, with scores of 3.5 or higher considered relatively safe.

The team evaluated airlines on five weighted factors: the number of water quality violations per aircraft under federal rules, maximum contaminant level violations for E. coli, the percentage of samples testing positive for coliform bacteria, the frequency of public notices related to water issues, and how often onboard systems were disinfected and flushed.

In total, 949 of the sampled locations, or about 2.7 percent, tested positive for total coliform bacteria, a sign that harmful pathogens could be present. Regulators also logged 32 E. coli violations across the 21 carriers studied during the three year period.

Those findings build on concerns first raised in a 2019 airline water study that reached similar conclusions about inconsistent water quality in the air and limited enforcement on the ground. While the new report points to improvements at some carriers, it concludes that millions of passengers a year may still be exposed to potentially unhealthy water when they wash their hands, brush their teeth or accept a cup of tea on board a flight.

American Airlines scores worst among major carriers

Among the 10 major U.S. airlines reviewed, American Airlines emerged as the lowest scoring large carrier for onboard water safety. The airline received a Water Safety Score of 1.75, earning a grade of D from the researchers. The report notes that American logged the highest number of E. coli related issues among the major airlines, a key factor that dragged its score down.

American was not alone at the bottom of the list. JetBlue Airways, which has long marketed extra legroom and customer friendly cabins, also posted a low grade, with a Water Safety Score of 1.80, another D. Spirit Airlines, which ranked worst in the 2019 water quality report, fared somewhat better this time but still landed in the D range among big carriers, reflecting lingering concerns about its onboard water systems.

By contrast, Delta Air Lines topped the rankings with a perfect score of 5.00 and an A grade, recording no maximum contaminant level violations for E. coli during the study period and maintaining a frequent disinfection and flushing schedule for its aircraft water systems. Frontier Airlines followed closely with a 4.80, also an A. Alaska Airlines rounded out the top tier of major carriers with a score of 3.85, earning a B.

Regional airlines show even deeper problems

While the headline findings focus on large national brands, the study suggests that travelers may face even higher risks on regional carriers that operate shorter domestic routes under familiar names like American Eagle, United Express or Delta Connection. Nearly all of the 11 regional airlines evaluated received low Water Safety Scores, indicating poor compliance with federal standards and frequent contamination issues.

Mesa Airlines, which flies for several major brands, posted the lowest overall score of any carrier in the study, with a 1.35 and a failing grade of F. CommuteAir, another regional operator, scored 1.60, or a D, and was flagged for an especially high rate of coliform positive samples, with roughly one third of its tested locations showing signs of bacterial contamination. The report describes those figures as alarming, given that regional jets and turboprops often serve smaller communities where passengers may have fewer flight options.

One notable exception was GoJet Airlines, which flies regional routes for United and Delta. GoJet earned a score of 3.85 and a B grade, placing it on par with Alaska and making it the best performing regional airline in the study. Even so, researchers cautioned that regional operations as a group remain a weak link in the national aviation water safety chain and require closer oversight.

Why airplane water is so vulnerable to contamination

The safety of drinking water at 35,000 feet is governed by a patchwork of regulations and operational realities that researchers say create ideal conditions for microbial growth. Aircraft potable water tanks are filled on the ground at airports around the world, often from multiple municipal or airport based systems in a single day. The water travels through hoses, trucks and transfer carts before reaching the plane, and each step introduces potential contamination risks if equipment is not properly cleaned and maintained.

Once on board, water sits in enclosed tanks and narrow lines that run through the fuselage. As the aircraft cycles through rapid altitude and temperature changes and endures mechanical vibrations, biofilms and bacteria can build up, particularly when planes sit idle for extended periods between flights. If airlines fail to flush and disinfect those systems frequently enough, or if quality checks are skipped, coliform bacteria and even E. coli can survive and multiply inside the system.

The U.S. Environmental Protection Agency adopted a specific Aircraft Drinking Water Rule in 2011 to address those challenges. The rule requires airlines to test water tanks for coliform bacteria and possible E. coli, disinfect and flush each aircraft tank several times per year or test monthly if maintenance is less frequent, and issue public notices when contamination is detected. Yet compliance relies heavily on self monitoring and reporting, and the rule was designed for an industry where aircraft routinely draw water from a variety of locations with varying oversight.

Regulators face criticism over weak enforcement

Beyond individual airline scores, the 2026 study delivers a pointed rebuke to federal regulators. The authors award what they call a symbolic Shame On You label to the Environmental Protection Agency, arguing that the agency has failed to aggressively enforce its own aircraft water standards or impose meaningful penalties when carriers fall short. Civil fines for violations remain rare, the report notes, even when repeated contamination issues are documented across an airline’s fleet.

Responsibility for safe water aloft is split among several agencies. The EPA oversees the quality of water supplied to aircraft and the operation of onboard water systems under the Aircraft Drinking Water Rule. The Food and Drug Administration regulates water used in food and beverage preparation, and the Federal Aviation Administration oversees airline maintenance programs, including potable water equipment. Public health advocates argue that this shared framework can diffuse accountability and slow responses when problems arise.

The EPA has previously acknowledged that aircraft public water systems pose unique compliance challenges, including rigorous schedules, multiple boarding locations and the use of temporary hookups and transfer equipment. In the mid 2000s, the agency found widespread noncompliance with traditional drinking water regulations among air carriers, a finding that helped prompt development of the aircraft specific rule. The new study suggests that more than a decade later, substantive gaps remain in how that rule is enforced.

What the findings mean for travelers

For passengers, the most immediate consequence of the report is a new wave of caution about routine inflight habits. The Center for Food as Medicine and Longevity is blunt in its recommendations: Travelers are advised to never drink water on a plane unless it comes from a sealed bottle, regardless of the airline’s ranking. That guidance extends to coffee, tea and other hot drinks that rely on the aircraft’s tap water, as well as to ice that might be made or handled using onboard supplies.

The study also urges fliers to be wary of using tap water from lavatory sinks for handwashing or toothbrushing. Instead, passengers are encouraged to pack travel size hand sanitizer containing at least 60 percent alcohol and to wash their hands with clean water in the airport before boarding. Many health experts suggest bringing an empty reusable bottle through security and filling it at an airport fountain or refill station, then keeping that as a primary water source during the flight.

For frequent travelers and families with young children or people with compromised immune systems, the findings may prompt a reevaluation of what constitutes a safe beverage onboard. While the absolute risk of serious illness from a single cup of tea at altitude remains relatively low, the researchers argue that the combination of documented violations and inconsistent maintenance makes avoidance a prudent choice, especially when safer alternatives are readily available.

Airlines respond as scrutiny intensifies

The publication of the latest water rankings has already triggered questions for carriers that received low scores, particularly American Airlines and JetBlue. Airlines typically point to their internal testing programs and compliance records to reassure customers that onboard water meets federal standards. They also note that hot beverages are prepared at temperatures that can help kill some bacteria, though experts say that alone may not be enough to fully address contamination risks linked to biofilms and residual organisms in the system.

Carriers at the top of the list, including Delta, Frontier and Alaska, are likely to highlight the study as evidence that their maintenance schedules and operating practices are working. Delta’s perfect score, achieved despite operating a far larger fleet than its closest rivals, is portrayed in the report as a sign that scale does not have to compromise water safety. Frontier, known as an ultra low cost carrier, may also use its strong performance to counter perceptions that budget airlines routinely cut safety related corners.

Whether the rankings lead to lasting operational changes across the industry remains unclear, but the report adds pressure on airlines and regulators at a time when passenger expectations about hygiene are already heightened. For now, researchers and consumer advocates say the safest move for travelers is to assume that aircraft tap water could be compromised and to plan accordingly on their next trip.