updated 1/8/2009 5:41:48 PM ET 2009-01-08T22:41:48

The Environmental Protection Agency is taking a second look at its decision not to limit the amount of a toxic rocket fuel ingredient allowed in drinking water.

Late last year, the agency proposed not setting a drinking water standard for perchlorate, which has been found in at least 395 sites in 35 states at levels high enough to interfere with thyroid function and pose developmental problems in humans.

At the time, the EPA said setting a standard would do little to reduce risks to public health.

The agency said Thursday it would postpone making a final decision until the National Academy of Sciences studies the matter. In the meantime, it recommended that levels of perchlorate in drinking water not exceed 15 parts per billion parts of water.

That level is lower than a threshold considered by the EPA late last year, but still much higher than standards set by states. In 2007, California adopted a drinking water standard of 6 parts per billion. Massachusetts wants the concentration of perchlorate in water to be no greater than 2 parts per billion.

"This is a sensible step for protecting public health and preserving regulatory options as the science of perchlorate is reviewed," Benjamin Grumbles, the agency's assistant administrator for water, said in a statement.

The EPA's own advisers had urged the agency to keep perchlorate on the list of water contaminants that may require future regulation. An inspector general's report issued late last year also faulted the agency for how it evaluated the risk the chemical poses to human health when it proposed in October not to set a limit for drinking water.

The decision on perchlorate has been pending for years, as the Pentagon and EPA tussled over the issue. The action Thursday will delay it further, and punt the decision to the incoming Obama administration.

The Defense Department used perchlorate for decades in testing missiles and rockets, and most perchlorate contamination stems from defense and aerospace activities.

The Pentagon could face liability if EPA sets a national drinking water standard that forces water agencies around the country to undertake costly cleanup efforts. Defense officials have spent years questioning EPA's conclusions about the risks posed by perchlorate, but they have denied trying to influence EPA's decision.

Environmentalists who had been critical of the earlier decision hoped the delay would lead to EPA regulation but still expressed concern.

"There was new science that they hadn't seriously looked at. There were bodies that were supposed to give input. It was a premature action," said Lenny Siegel, director of the Center for Public Environmental Oversight in Mountain View, Calif. "The concern is that this ... again it does not take into account the most robust science on the subject."

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