Four coal-fired power plants in Western Pennsylvania have landed on this year's ranking of the 50 dirtiest for emissions of the greenhouse gas carbon dioxide, according to a new report by a nonprofit environmental group.

FirstEnergy Corp.'s Bruce Mansfield plant in Shippingport, Beaver County, ranked 17th in the nation for emitting more than 17.3 million tons of carbon dioxide last year, said the Environmental Integrity Project, a Washington, D.C.-based organization that advocates for better enforcement of existing federal and state anti-pollution laws.

Also on that list are Reliant Energy's Conemaugh Generating Station, at 29th, and Keystone power plant, at 42nd, and Edison International's EME Homer City plant, at 43rd. All three are in Indiana County.

Pennsylvania, with four power plants on the "50 dirtiest" list for total carbon dioxide emissions, tied Indiana for second most. Both ranked behind Texas, which had five plants on the list.

The annual listing and report by Environmental Integrity, based on 2006 emissions data from 378 of the largest fossil fuel-burning power plants nationwide, also ranks their sulfur dioxide, nitrogen oxide and mercury emissions. The rankings reflect both the total amounts of emissions and the emission rate, which is the amount of emissions per megawatt-hour of electricity produced.

The ratings show that last year's trend toward reduced sulfur dioxide and nitrogen oxide emissions from power plants is continuing. Emissions of mercury, a dangerous neurotoxin that can harm pregnant women and children who eat fish or seafood, are, however, holding steady at 48 tons nationwide.

But the report's primary focus, on the electric power industry's carbon dioxide emissions, notes they are unchecked and unregulated, and could rise 34 percent by 2030 as up to 159 new coal- and natural gas-fired power plants come on line.

Carbon dioxide is a greenhouse gas that has been building up in the atmosphere and has been linked to climate change and global warming.

"While Congress is poised to seriously consider legislation to limit the greenhouse gases that made 2006 the hottest year on record, the electric power industry is racing to build a new fleet of coal-fired power plants that rely on conventional combustion technologies that would only exacerbate global warming," said Ilan Levin, an Environmental Integrity attorney and lead author of the report released yesterday.

Seven of the new coal-fired power plants are planned for Pennsylvania, which already has 36 coal-fired facilities.

Jan Jarrett, vice president of Citizens for Pennsylvania's Future, a statewide environmental group, said some power plant emissions are declining, but strong pollution controls are still needed.

"Reductions in sulfur dioxide and nitrogen pollution from power plants are a direct result of a strong federal Clean Air Act, but it took more than 30 years," Ms. Jarrett said.

"That's one of the reasons we fought so hard for the Pennsylvania mercury rule, which will cut mercury pollution by 90 percent by 2015. We knew the federal government wasn't about to take action, so the state had to. Now is the time to demand cuts in carbon dioxide pollution to fight global warming."

Chris Eck, a FirstEnergy spokesman, said the Bruce Mansfield power plant is one of the 20 biggest in the nation, producing 2,460 megawatts of power, so its high tonnage of carbon dioxide and nitrogen oxide emissions -- it ranks 19th nationally -- are not out of line with its output.

He said the plant's emission rate -- that is the amount of emissions per megawatt-hour of electricity produced -- is "very competitive," ranking 243rd for carbon dioxide and 166th for nitrogen oxide.

Nationwide, electric power plants emit about 40 percent of man-made carbon dioxide, about two-thirds of sulfur dioxide, 22 percent of nitrogen oxides and a third of all mercury emissions.

The full Environmental Integrity Project report can be viewed online at www.dirtykilowatts.org.