Drinking water aboard the nation's airliners is getting worse, not better, despite government-ordered sanitation improvements, the Environmental Protection Agency (search) said Wednesday.

About one in six airliners in the latest round of tests conducted in November and December had drinking water that failed to meet federal safety standards, EPA said. Similar tests in August and September showed the water in one in eight aircraft had coliform present in the drinking water. The current tests showed one in six airliners had drinking water below safety standards.

Nearly 170 passenger planes were randomly selected at 14 U.S. airports.

The latest round of testing produced positive results for presence of the coliform bacteria (search) found in planes ranging from small commuter aircraft to jumbo jets. Coliform is usually harmless by itself. None of the planes had E. coli bacteria (search), which can cause gastrointestinal illness.

"It's an issue that's of concern," said Thomas V. Skinner, acting assistant administrator for EPA's Office of Enforcement and Compliance Assurance (search). "It's not an indication that anyone needs to panic."

Despite the increased rate of aircraft testing positive over a previous round of testing, Skinner said he "would still maintain that the vast majority of planes do not come up positive."

He said the government does not plan a third round of tests.

EPA's tests last August and September found coliform bacteria on 20 of the 158 randomly selected aircraft. Two planes then also tested positive for E. coli bacteria, which can produce diarrhea and nausea. About 73,000 cases of E. coli infection are reported in the United States each year.

EPA advised passengers with compromised immune systems or others concerned to ask for canned or bottled beverages and refrain from drinking tea or coffee unless made with bottled water.

Combining the two rounds of testing on 327 aircraft last year, EPA officials noted that about 15 percent of the planes had been found with coliform bacteria.

EPA and 12 major airlines agreed in November on a program aimed at improving sanitation. It included more testing of aircraft. Airliners would be disinfected within 24 hours if coliform bacteria were discovered, unless the agency granted an extension because the plane was outside the United States. Passengers would find signs posted in lavatories and galleys.

Signing agreements with EPA were Alaska Airlines, Aloha Airlines, American Airlines, America West, ATA Airlines, Continental Airlines, Hawaiian Airlines, JetBlue, Midwest Airlines, Northwest Airlines, United Airlines and U.S. Airways.

Two additional airlines, Delta Airlines and Southwest Airlines, are currently negotiating separate agreements with EPA.