Pennsylvania’s coal-fired power plants would have to cut mercury emissions 90 percent within a decade under tougher-than-federal mercury regulations proposed for the fourth-largest coal-producing state. The plan was called overzealous by a power industry group.

The state Department of Environmental Protection said last year that it would try to write a mercury rule that would be tougher than the new federal rule, which projects a 70 percent average reduction in mercury emissions by 2018, at the earliest.

Pennsylvania sued over the federal rule, contending that it has loopholes that could allow the state’s coal plants to avoid cleaning up and discriminates against the type of coal found in Pennsylvania.

Four other states regulate mercury emissions, but none mines more coal than Pennsylvania, which trails only Wyoming, West Virginia and Kentucky in production.

Environmental Protection Secretary Kathleen McGinty said she believes the proposed rule balances the public health concerns against the economic interests of coal and power producers.

“The hope is that we very substantially take a bite out of a pollutant that is a very serious problem, especially in terms of infants and unborn children and pregnant women,” McGinty said.

Mercury becomes airborne in the coal-burning process and works its way up the food chain, accumulating in plants, then fish and humans. Mercury is a major cause of fish consumption advisories in the Bay watershed. Children and fetuses are particularly vulnerable to effects of the toxic metal, which can damage the development of the nervous system, according to the EPA.

Pennsylvania, also one of the nation’s top power-producing states, generated 5.7 tons of airborne mercury in 2003, the second most in the country, according to federal data. About three-fourths of that airborne mercury came from the state’s three dozen coal-fired power plants, according to the figures.

The federal rule requires an eventual mercury reduction of 86 percent from power plants in Pennsylvania, although DEP officials question whether that is possible. Under the proposed state rule, Pennsylvania’s three dozen coal-fired power plants would have to cut mercury emissions by 80 percent by 2010 and by 90 percent in 2015.

While the federal rule encourages switching to a lower-sulfur coal, the state rule would give preferential treatment to plants that burn the higher-sulfur bituminous coal found in Appalachian states.

The state’s rule also would bar a complicated credit-trading program that federal mercury regulations allow. The credit-trading lets some plants put off expensive pollution-control upgrades by purchasing “credits” that cleaner plants earn by exceeding the requirements.

Doug Biden, president of the Electric Power Generation Association, a power industry group in Pennsylvania, said the Pennsylvania rule is the result of “overzealous environmental regulation” that makes little sense.

Biden said the state’s rule could leave Pennsylvania’s coal plants at a competitive disadvantage with less-regulated plants in other states, and be a crushing financial blow to the smaller ones. Also, it provides little flexibility for power plants struggling to meet the new requirements at a time when coal is a cheaper source of electricity than natural gas, he said.