A surprising number of drinking water systems on domestic and foreign commercial aircraft tested this summer by the government did not meet federal standards because they were contaminated with potentially harmful bacteria, regulators said on Monday.

The Environmental Protection Agency suggested passengers with compromised immune systems - usually the elderly, very young or those who are sick - not wash with water from the tap or drink beverages like coffee and tea that are brewed or mixed on flights.

“This is preliminary and we want to follow up on this and learn more,” cautioned Thomas Skinner, head of the EPA’s enforcement office.

Airlines defended aircraft water quality and said the government’s conclusions were drawn from samples taken from only a fraction of the thousands of aircraft in the fleet.

The industry also questioned the government’s testing methods and whether samples were adequately protected from contamination from other sources.

The agency said it was satisfied with its protocols and has expanded its sampling. It is also working to strike an agreement with carriers for proper oversight, maintenance and sanitation of water delivery equipment and onboard storage tanks.

No airline has been cited for violating federal clean water standards.

The EPA randomly tested drinking water from 158 U.S. and foreign aircraft during August and September.

The results showed more than 12 percent tested positively for total coliform bacteria. Water on two of those planes contained E. coli, a potentially deadly bacterium commonly associated with food poisoning. The agency is retesting those aircraft.

Skinner and other EPA officials called the results surprising, and determined it was prudent to alert the public pending further analysis.

None of the airlines were identified, but Skinner said contamination occurred on a cross-section of aircraft and flights. The airlines were aware of the tests while they were being conducted. Some of the flights originated overseas.

Skinner said there was nothing immediately apparent to link the contamination, like common airlines, aircraft, drinking water sources, distribution outlets or maintenance practices or companies.

“There are no reported cases of illnesses due to aircraft drinking water. And there were no adverse health effects found in the EPA study or any other federal study,” the Air Transport Association said in a statement.

The trade group, which represents major U.S. airlines, suggested the EPA results conflict with previous government and industry studies showing airline water free of contaminants.

Airline water systems are pumped into aircraft tanks, which feed both bathroom and galley taps.