Drinking water on commercial airliners will be checked more frequently after the Environmental Protection Agency (search) found evidence of harmful bacteria in the water of one of every eight planes tested.

Twelve major airlines have agreed to sanitation improvements and increased testing of drinking water aboard aircraft. EPA also said Tuesday it would conduct random water quality tests on 169 domestic and international passenger aircraft at 14 airports throughout the United States and publish the results by the end of the year.

Thomas V. Skinner (search), acting assistant administrator for EPA's Office of Enforcement and Compliance Assurance, said in a statement Tuesday that more frequent testing will mean "increased protection to the flying public."

Airliners would have to be disinfected within 24 hours if coliform bacteria were discovered, unless the agency granted an extension because the plane was outside the United States. In the meantime, passengers would find signs posted in the lavatories and galleys of affected aircraft.

Two months ago, EPA tested drinking water aboard 158 randomly selected domestic and international passenger aircraft and found that 20 had drinking water that did not meet federal safety standards.

The planes — small commuter aircraft to jumbo jets — returned positive results for coliform bacteria, usually harmless by itself but an indicator of the possible presence of other harmful organisms. Two planes tested positive for E. coli bacteria, which can cause gastrointestinal illness.

EPA advises passengers with immune system problems to avoid drinking water from airplane galleys or lavatories.

But the airlines said they are confident their drinking water is safe, and they believe the number of airplanes that failed the agency's safe water test is closer to one in 20.

The Air Transport Association, which represented the airlines in Tuesday's agreement, said in a statement, "Our members wanted to address once and for all questions the EPA raised about airline drinking water." Still, the association said, the airlines believe "aircraft drinking water is just as safe as the municipal water systems that supply it."

The new agreement could be costly to the industry, in part because some planes will have to be flown to airports capable of the flushing and testing required.

The agreement requires the airlines to analyze possible sources of contamination that exist outside aircraft and to tell the government how they get drinking water from foreign public water suppliers not regulated by EPA.

Under the agreement:

—Airlines must disinfect and flush each airplane's potable water system quarterly.

—Airlines must flush out trucks, carts and hoses that carry the water monthly.

—Airlines must notify EPA immediately when an airplane tests positive for coliform bacteria; EPA will ensure the problem is corrected.

—EPA will meet with the airlines quarterly over the next year to determine whether changes in the process are needed.

—After the first year, the airlines and EPA will meet for 30 days to decide on the process for the next year.

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.

Separate agreements are being negotiated with Delta and Southwest airlines, EPA officials said. The agency said it also is working with regional and charter airlines to improve drinking water quality.