Coal-burning power plants in Pennsylvania are among the biggest emitters of sulfur dioxide and mercury pollution in North America, according to a report released yesterday by the Commission for Environmental Cooperation.

The report is the first comparison of emissions from more than 1,000 fossil fuel power plants in the United States, Canada and Mexico. It found that a small percentage of the plants release much of the pollution and that there are dirty power plants in each nation.

Allegheny Energy's Hatfield's Ferry power plant in Masontown, Greene County, and Reliant Energies Inc.'s Keystone power plant in Shelocta, Armstrong County, are the second- and third-biggest emitters of sulfur dioxide in the United States.

Two plants in Mexico that burn heavy oil instead of coal are even bigger emitters of sulfur dioxide.

Four of the top eight mercury emitters in the United States are in Pennsylvania: Keystone (ranked 2nd); Edison International's EME Homer City in Indiana County (3rd); PP&L's Montour Electric in Danville, Montour County (5th); and First Energy Corp.'s Bruce Mansfield plant in Shippingport, Beaver County (8th).

Pennsylvania's power plants don't make the top 20 for emissions of carbon dioxide and nitrogen oxides, which have been controlled for years under a regional air pollution control strategy established by the Ozone Transport Region. Pennsylvania is in the region; Ohio and states to the west are not and power plants there occupy nine of the top 10 spots for nitrogen oxide emissions.

"The numbers look very hazy for the United States," said Frank O'Donnell of Clean Air Watch, an environmental organization. "Our emissions are very high and underscore that relatively few power plants are using modern pollution control equipment 35 years after the passage of the Clean Air Act."

Nitrogen oxide emissions react with sunlight to produce ground level ozone, the major component of smog, which can cause breathing problems, particularly for people with chronic lung disease, the young and elderly.

"The numbers show that those kinds of [pollution control] measures work to improve emissions," said Paul Miller, co-author of the report for the commission, a trilateral, intergovernmental organization created in 1994 under the North American Free Trade Agreement.

Miller said the report's data will be used to start tracking emissions trends and to support possible international emissions trading programs to reduce pollutants.