Maryland may require billions of dollars in improvements to coal-fired power plants in metropolitan Baltimore and the District of Columbia in what’s described as the most sweeping air pollution control measure ever enacted in the state. Power plants would have to sharply reduce pollutants such as nitrogen oxide—a major Bay pollutant—and sulfur dioxide by 2010, changes that authorities said could slash some harmful power plant emissions by up to 85 percent.

The proposed change, announced by Gov. Robert Ehrlich, comes after the legislature last year considered, but could not agree on, similar air pollution efforts. Ehrlich’s version would not affect carbon dioxide emissions from the six power plants, a feature of the bill he opposed last session. He said his version would make huge strides in air quality without putting an undue burden on power suppliers.

The price tag for the cleaner standards could run into the billions and will be paid by the power companies, said Tom Snyder, director of the state’s Air and Radiation Management Administration.

One power company that would be affected, Baltimore-based Constellation Energy Group Inc., warned that the change could mean higher bills for consumers.

“Without the flexibility for cost-effectively meeting new targets, the price paid by Maryland energy consumers could unnecessarily increase,” said a statement put out by Constellation. The company also said it favored regional or national rule changes, not state-level changes.

The other company with power plants that would be affected, Atlanta-based Mirant Co., did not immediately respond to the proposal.

Jonas Jacobson, deputy secretary of the Maryland Department of the Environment, said the tougher rules aren’t an option if the state is to have a chance of meeting future federal air quality standards.

“What we realized was that there was a gap in the benefits that we would get from the federal guidelines and what we needed to meet attainment of clean air standards by 2010,” Jacobson said.

Environmental experts said they were happy to see changes made at all. “It’s a major step,” said Donald Boesch, director of the University of Maryland’s Center for Environmental Science. Of the tougher emissions standard, he said, “It’s something we had to do in the long run and we had to accelerate it.”

Others weren’t so happy. The Chesapeake Climate Action Network, a nonprofit advocacy group, said the lack of carbon dioxide controls was a major flaw in the plan. Mike Tidwell, the group’s director, said carbon dioxide controls are crucial in fighting ozone pollution and global warming.

Ehrlich’s proposal must go to a review committee of state delegates and senators, but a vote by the full legislature isn’t required for the rule changes to take effect.

About 60 percent of Maryland power comes from coal-powered plants. Two coal-fired power plants in western Maryland would not be affected because they aren’t in an area that falls below federal clean air standards.