A new smokestack is not usually cause for celebration among environmentalists. But the 400-foot stack spouting white clouds at Brandon Shores power plant represents a quantum leap in cleaning Baltimore's air, not another source of pollution.

Constellation Energy has just completed work on $875 million worth of pollution "scrubbers" at its 26-year-old coal-fired power plant on the Patapsco River. One of the plant's two steam-generating units resumed operation with the new air-quality controls in December, and the second is cranking up now. The white clouds rising from the stack are almost entirely water vapor. A pair of 700-foot stacks nearby, which until recently belched toxic, acidic smoke from the power plant, are quiet.

Activists not prone to praising power companies are grudgingly expressing appreciation, though they point out that the plant improvements were made to comply with a nearly four-year-old state law intended to make the air healthier to breathe.

Woody Bowen, 67, who lives nearby, calls the three-year pollution-control project at the plant "a major, major thing." The president of the Olde Brooklyn Park Improvement Association says he's looking forward to "a cleaner sky and less contaminants coming down in the community."

There's a lot to clean. Until recently, Brandon Shores, along with Constellation's H.A. Wagner power plant on the same 360-acre riverfront tract, have together been the nation's leading emitter of hazardous air pollutants, according to data from the Environmental Protection Agency.

But that's likely to change with the installation of the twin scrubbers at Brandon Shores and pollution controls put in at Constellation's other coal-burning plants in the area. The Baltimore-based power company has invested more than $1.5 billion to comply with Maryland's Healthy Air Act, which when it was passed in 2006 was billed by state officials as the toughest power-plant pollution law on the East Coast.

Under the law, the state's power plants were required to reduce harmful emissions by 70 percent to 80 percent by this year, and by 75 percent to 90 percent by 2013. Targeted are releases of nitrogen oxides, sulfur dioxide and mercury - byproducts of burning coal that contribute to environmental and health problems in the state.

Nitrogen oxides contribute to ground-level ozone pollution or smog that can make hot summer air difficult or painful to breathe. They harm water quality in the Chesapeake Bay as they drop from the air. Sulfur dioxide is a major source of fine-particle pollution that can cause breathing difficulties or premature death.

Mercury is a toxic metal that, in small doses, can damage the brain, nervous system and other organs. It accumulates in fish tissue, prompting state health officials to warn against eating too many fish caught locally.

Power plants are a leading source of such pollutants. Activists pressed in 2006 to get Maryland to crack down and bring the state's air quality in line with federal health standards.

George S. "Tad" Aburn, director of air management for the Maryland Department of the Environment, calls the Healthy Air Act "the most significant pollution-control program we've ever implemented."

He recalled that Constellation and Atlanta-based Mirant Corp., which operates coal-burning plants elsewhere in the state, had resisted the legislation and argued that the reductions could not be achieved according to the timetable set in the law.

"All those horror stories did not happen," Aburn said. Though state regulators frequently spar with power plant owners and fine or sue them, Aburn noted, "in this case, Constellation and Mirant did a very good job."

Six scrubbers were required. Besides the two installed at Brandon Shores, Mirant put in "flue-gas desulfurization" facilities at its Chalk Point, Dickerson and Morgantown power plants. The company said those and other pollution controls cost it nearly $1.7 billion.

Some environmentalists say it's about time. Scrubbers have been required for years on new coal-burning power plants elsewhere, and even on some particularly large or polluting older ones. But until the Healthy Air Act was passed in 2006, none had been ordered in Maryland.

"This means Maryland will jump from the back of the pack to kind of the head of the class," said Eric Schaeffer, director of the Environmental Integrity Project, a Washington group.

Installing the scrubbers was a mammoth undertaking. Two thousand contractors and construction personnel worked on the project, with up to 1,600 of them at the plant at one time, said Joseph Kappes, the construction oversight manager.

"It was like building a small city," Aburn said.

The scrubbers are the last and latest line of control equipment put on the plant to capture its air pollution. Pollutants from the hot exhaust from the coal-burning boilers are filtered out, with some being converted into gypsum, a nontoxic material used in wallboard. Then the treated exhaust gas, with the vast majority of its pollution removed, is released via the scrubber stack.

"We don't expect them to close down," said Mary Rosso, a community activist and former state delegate from Anne Arundel County. "We just expect them to do the most they can, considering they are there and they're the highest air polluters."

Residents near the plant are still jousting with Constellation over its disposal of the ash left after burning coal at the Brandon Shores and neighboring Wagner plants. The company paid a $1 million fine and settled out of court for $54 million more a couple of years ago for contaminating wells in the Gambrills area with ash dumped into an old quarry.

Now, most of the ash - which can contain toxic metals and other contaminants - goes into making cement and is put to other beneficial uses, the company says, while about 20 percent of it is hauled by truck to a landfill in Virginia. The company is seeking state approval to dispose of ash in an industrial landfill in nearby Hawkins Point.

Some fear that the pollutants now being removed from Maryland's air might wind up fouling its land or water. Constellation officials say the gypsum produced by the scrubbers is free of contaminants and suitable for use in building. The water used in the scrubbers is treated wastewater from Anne Arundel's Cox Creek sewage plant, says plant manager John N. Kusterer, and is treated again to state specifications before being discharged into the Patapsco.

Constellation says the new controls have made Brandon Shores, one of the dirtiest coal-burning power plants in the country, into one of the cleanest of its size. When running at capacity, the plant can generate 1,300 megawatts of electricity, enough to power 1 million households for a year.

"We always understood that we owed a responsibility to Maryland," said John Long, president of Constellation's power generation subsidiary.