The Environmental Protection Agency has determined that more than half of all freshwater fish it sampled from America's lakes could be unsafe for women of childbearing age to eat twice a week, according to data disclosed by environmental groups.

More than three-quarters of the fish sampled also had mercury levels that may be unhealthy for children younger than 3. The data, collected between 1999 and 2001 on 2,547 fish from 260 lakes, are part of the first-ever nationwide study the EPA has conducted on freshwater fish in an ongoing four-year project.

"It's a public health imperative to reduce mercury emissions as quickly as possible," said Emily Figdor, a policy analyst for Clear the Air, which compiled the EPA findings. The new numbers, which EPA released as raw data in the past year, represent the latest evidence that mercury emissions pose a public health threat, the environmentalists said.

In March, the EPA and the Food and Drug Administration warned pregnant and nursing women and young children against eating more than a small amount of canned albacore "white" tuna once a week because of mercury contamination, based on analyses of commercial saltwater fish sampled from the marketplace.

For freshwater fish, federal officials advised consumers to check local health advisories. As of 2002, 43 states had warned residents to limit how much freshwater fish they consume, restrictions that encompass 30 percent of the nation's lakes and 13 percent of its rivers.

Jim Pendergast, chief of the EPA Office of Science and Technology's health protection and modeling branch, said the agency has yet to establish a safe limit for freshwater fish and said the mercury levels outlined in yesterday's report will not necessarily make consumers sick.

The EPA has determined there is no health risk for women and children eating less than 0.1 micrograms of mercury per kilogram of body weight per day, but less than half the fish in the new survey met that standard, assuming two fish meals a week.

Mercury, a metal, is toxic and can cause neurological and developmental problems in children.

Some mercury exposure stems from industrial air pollution that gets into water and the food supply, in part because it builds up in predator fish. Coal-fired power plants rank as the greatest U.S. source of mercury pollution, according to the EPA, and environmentalists say the Bush administration is not doing enough to curb plants' emissions.

President Bush has proposed regulations that would reduce pollution from mercury plants by 70 percent by 2018. But Figdor and others cited a federal Energy Information Administration study in May that projected the plan would not meet this goal until after 2025.

"The rule doesn't come close to doing what it needs to do to solve the problem of mercury contamination in our lakes," said Clear the Air's director, Angela Ledford.

EPA spokeswoman Cynthia Bergman said the agency is still assessing what its plan would achieve.

The plants "must be cleaned up," Bergman said. "We are proposing to do that for the first time ever in the history of the EPA."

Industry officials countered that coal-fired plants account for just 1 percent of global mercury emissions, and that some academic studies suggest much of the mercury in the environment is naturally generated.

"No matter how great or small the reductions we make in our emissions, there will be very little measurable benefit to public health," said Edison Electric Institute spokesman Dan Riedinger, whose group represents 190 utilities. "This is not an excuse for inaction, but these groups are overpromising the public health benefits that are not going to be achieved."

There is some anecdotal evidence that reducing industrial mercury pollution translates into cleaner fish, said Thomas D. Atkeson, mercury coordinator for Florida's Department of Environmental Protection. Mercury pollution dropped by 90 percent in South Florida since medical waste and municipal solid-waste incinerators there installed new controls in early 1990s, he said, as there are no coal-fired plants in the area.

Mercury levels in largemouth bass and wading birds declined 80 percent during that same period, and local officials have eased restrictions on eating fish from the north and central Everglades.

"None of us thought we would live to see levels of mercury concentrations in the Everglades come down as quickly and as sharply as they have," Atkeson said.

Researcher Karl Evanzz contributed to this report.