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Massachusetts will have the nation's most stringent limits on mercury emissions from power plants, the governor's office announced yesterday. State environmental groups heralded the announcement, saying the final regulations were stricter than the draft plans issued last fall.

The new regulations require the state's four coal-burning power plants, including two in Somerset, to reduce their mercury emissions by 75 percent by 2010, and nearly eliminate mercury releases over the long term.

Officials at Brayton Point Power Station in Somerset, said they plan to comply with the regulations.

High mercury levels in the bloodstream can threaten children, causing neurological difficulties and learning disabilities, according to Jed Thorp, of environmental watchdog group Clean Water Action.

The contaminant can be stored in fish, and passed to pregnant or nursing women who eat the fish, and then pass it on to their children.

The Centers for Disease Control and Prevention and the federal Environmental Protection Agency estimate that in the United States, 1 out of 6 women of childbearing age has unsafe levels of mercury in their blood.

Because the pollutant can be passed onto infants, state health officials advised in July 2001 that pregnant women, nursing mothers, and young children not consume freshwater fish from Massachusetts, due to the health risk posed by mercury contamination.

The limits on mercury are the latest addition to 2001 state rules aimed at reducing pollutants that cause acid rain, smog and regional haze.

According to the Executive Office of Environmental Affairs, the mercury regulations limit emissions based upon the amount of electricity generated by each facility.

By 2010, the plants must prevent 85 percent of the mercury emissions from escaping into the atmosphere. By 2012, that must be improved to a 95 percent capture rate.

A spokeswoman for Brayton Point Power Plant said yesterday the impact of the regulations on that plant were still unknown. "We'll work to comply with whatever the final regulations require," said Natalie Wymer, spokeswoman for Brayton's parent company, National Gas & Energy Transmission.

Wymer said it is too early to know what the cost of the regulations will be at Brayton Point.

Thorp at Clean Water Action said the new regulations will require sizeable on-site reductions from Brayton Point. These regulations, unlike earlier draft plans, require deep reductions from the plant's coal exhaust, Thorp said. Earlier regulations would have allowed the plants to reduce mercury levels elsewhere in the plant.

A second Somerset plant, the former Montaup Electric Plant, will also be affected. The other two targeted plants are the Salem Harbor station in Salem, and the Mount Tom station in Holyoke.