EPA estimates that the proposed rule will reduce mercury emissions by 69 percent by 2018, said Kurt M. Knaus, spokesman for the Pennsylvania Department of Environmental Protection. But the earlier rule would have cut emissions by 90 percent by 2010, he said.

In a news release, DEP Secretary Kathleen A. McGinty said the current proposal "endangers pregnant women, children and others who are most at risk for health defects that can include brain and nervous system damage."

Mercury is released into the air when coal is burned. It falls to the ground and is washed or falls into streams, where it accumulates in the tissues of plants and animals. Human exposure comes primarily from eating fish.

The filing by Pennsylvania and the other states came as the time period for public comments on the proposed rule draws to a close tomorrow, Knaus said.

EPA will take the comments into consideration before issuing a final rule.

Scott Segal, director of the Electric Reliability Coordinating Council, said his group would be filing comments today in support of the regulation. The earlier regulation could have prompted companies to switch to natural gas rather than continue to use coal, which would have boosted costs to consumers, Segal said.

McGinty, who oversaw development of the Clinton administration's pollutant control proposal, said the current EPA proposal penalizes Pennsylvania coal. That's because the emissions standards are more stringent for the cleaner bituminous coal in Pennsylvania than they are for sub-bituminous coal mined in the West.

A study by the Centers for Disease Control and Prevention found that one in 12 women of childbearing age in the United States has mercury in her blood above levels that the EPA considers safe.

In March, the Food and Drug Administration and the EPA issued an advisory to pregnant women about the risks of mercury in fish and shellfish, offering tips on how to reduce the risks.