WASHINGTON - The Environmental Protection Agency, under pressure from the White House and the Pentagon, is poised to rule as early as today that it will not set a drinking water safety standard for perchlorate, a component of rocket fuel that has been linked to thyroid problems in pregnant women, newborns, and young children across the nation.

According to a near-final document obtained by the Washington Post , EPA's "preliminary regulatory determination" - which was extensively edited by White House officials - marks the final step in a six-year battle between career EPA scientists who advocate regulating the chemical and White House and Pentagon officials who oppose it. The document estimates that up to 16.6 million Americans are exposed to perchlorate at a level many scientists consider unsafe; independent researchers, using federal and state data, put the number at between 20 million and 40 million.

Some perchlorate occurs naturally, but most perchlorate contamination in US drinking water stems from improper disposal at rocket test sites, military bases, and chemical plants. A nationwide cleanup could cost hundreds of millions, if not billions, of dollars, and several defense contractors have threatened to sue the Defense Department to help pay for it if one is required.

The new EPA proposal - which assumes the maximum allowable perchlorate contamination level is 15 times above what EPA suggested in 2002 - was heavily edited by officials of the White House Office of Management and Budget who edited out key scientific passages and asked EPA to use a new computer modeling approach to calculate the chemical's risks. Under a process OMB initiated in 2004, federal agencies with an interest in chemicals such as perchlorate, such as the Defense Department, have additional opportunities to influence EPA's regulatory decisions before they become final: The Government Accountability Office reported this spring that the Pentagon had pressured EPA for several years not to regulate perchlorate.

"They have distorted the science to such an extent that they can justify not regulating" the chemical, said University of Massachusetts professor Robert Zoeller, an endocrinologist who specializes in thyroid hormone and brain development and who has a copy of the EPA proposal. "Infants and children will continue to be damaged, and that damage is significant."

Zoeller noted that scientific studies have shown a small reduction in thyroid function in infants can translate into a loss of IQ and an increase in behavioral and perception problems. "It's absolutely irreversible," he said. "Even small changes in thyroid functions early on have impacts on functioning through high school and even into people's 20s."

A reference to those studies in EPA's proposal was deleted by OMB officials.

The document states that establishing a drinking water standard for perchlorate "would not present a 'meaningful opportunity for health risk reduction for persons served by public water systems.' "

In a statement yesterday, EPA's assistant administrator for water Benjamin Grumbles said "science, not the politics of fear in an election year, will drive our final decision."

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