WASHINGTON — The Bush administration planned to issue the nation's first regulations to cut mercury pollution from electric utilities today, relying on a market trading system that gives companies 15 years to reduce it nearly by half. The Environmental Protection Agency's regulations are aimed at reducing levels of a toxic chemical that can severely damage nervous systems, especially in fetuses and children. They result from a lawsuit brought by an environmental group 13 years ago.

The EPA expects to reduce the current 48 tons a year of mercury from smokestacks of coal-burning power plants down to 31.3 tons in 2010, according to a copy of the rule provided Monday by environmental groups. The regulation would further reduce that to 27.9 tons in 2015, and to 24.3 tons in 2020. EPA officials did not dispute those numbers.

EPA spokeswoman Cynthia Bergman said the mercury rule at first "relies completely on the reductions" that power plants must make to comply with companion regulations issued last week to reduce interstate pollution from fine particles and smog-forming ground-level ozone.

The agency believes significant reductions in mercury will result as a "co-benefit" when plants install new equipment to reduce sulfur dioxide and nitrogen oxides, she said.

"While this rule is protective of public health, most of the mercury that creates health risks for Americans comes from fish contaminated from sources that we can't control," Bergman said Monday. "This is a global problem."

In the meantime, she said, pregnant women and women of childbearing age should heed government warnings to limit fish intake.

In written statements Monday, members of Maine's congressional delegation - Republicans and Democrats alike - criticized the rules.

Sen. Olympia Snowe, a Republican, called the rule "woefully inadequate and profoundly disappointing," and said it "will not address - and could even exacerbate - the formation of mercury hotspots."

Her Republican colleague, Sen. Susan Collins said that she "will review the proposal when it comes out, but I am concerned that it will not fix the core problem. I intend to continue to push for more stringent standards."

Collins was a co-sponsor of the Clean Power Act.

U.S. Rep. Tom Allen, a Democrat who represents Maine's 1st Congressional District, said the rule is "literally written by the polluters' lawyers and blatantly in violation of law." He said the EPA "has stained its own credibility."

Mercury concentrations accumulate in fish and work up the food chain, which has prompted most states to issue fish consumption advisories. Forty percent of mercury emissions come from the smokestacks of more than 450 coal-burning power plants, but those emissions have never been regulated as a pollutant.