E. coli is also associated with human waste.

"We take fecal contamination seriously. It can have an acute gastrointestinal impact," says Ben Grumbles of EPA's Office of Water.

Although the forms of coliform and E. coli found in the tanks don't usually make people sick, they are indicators that other organisms might be present that could pose a danger to public health.

Symptoms of such water-borne illness include diarrhea, nausea and vomiting.

But the public shouldn't overreact, says Phyllis Kozarsky, chief of travel health at the Centers for Disease Control and Prevention.

"We have not had reports of water-borne outbreaks on planes. People have not become ill, and we have no reports of illness," she says.

The samples were collected in August and September from a wide range of aircraft types and carriers.

The agency also tested for residual chlorine to determine whether the water systems had been disinfected.

The tests provide a statistically valid profile of aircraft water quality in the USA, the EPA says.

The agency says the data are preliminary, and it is undertaking a more thorough testing program to understand the full scope of the problem.

"We're pursuing what may be violations of our regulations and reviewing whether we need to strengthen our guidelines to carriers," Skinner says.

"We've notified the airline industry of our findings, and we're actively pursing additional ways to clean the water."

The Air Transport Association, the trade organization of the major U.S. airlines, disputed the findings.

The organization said that testing by the Food and Drug Administration as well as its own tests found that "airlines' drinking water was free of contaminants that might pose health risks," according to a press release.

"People have to use common sense and not have greater expectations than they should have. ... An airplane is not their home," the CDC's Kozarsky says.

But even home water might not be a good comparison.

Although the 13% failure rate in airplane water might seem high, the EPA's Grumbles says 10% of the U.S. population gets water from sources that do not meet federal water quality standards.