WASHINGTON (AP) — The Environmental Protection Agency is reviewing part of a controversial rule that sets the first federal standards to reduce toxic air pollution from power plants.

The rule was issued in December. It's aimed at curbing mercury and other toxic pollutants from coal- and oil-fired power plants. The Obama administration calls the rule a sensible step to reduce pollution, but Republicans have denounced it as a part of a "war on coal." The rule could force hundreds of the nation's oldest and dirtiest power plants to clean up or shut down.

The EPA said Friday it's reviewing an aspect of the rule that applies to future power plants. The technical review is intended to clarify how the new standards would apply to five plants proposed in Texas, Georgia, Kansas and Utah.

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  • Top 10 Most Polluting Countries

    We look at which 10 countries have the most CO2 emissions. Figure are preliminary 2010 numbers from the U.S. government's <a href="http://cdiac.ornl.gov/trends/emis/perlim_2009_2010_estimates.html" target="_hplink">Carbon Dioxide Information Analysis Center. </a> (Photo Getty Images)

  • #10 - Saudia Arabia

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 493,726 (Photo MARWAN NAAMANI/AFP/Getty Images)

  • #9 - Canada

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 518,475 (Photo MARK RALSTON/AFP/Getty Images)

  • #8 - Korea

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 563,126 (Photo CHOI JAE-KU/AFP/Getty Images)

  • #7 - Iran

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 574,667 (Photo FRED DUFOUR/AFP/Getty Images)

  • #6 - Germany

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 762,543 (Photo JOHANNES EISELE/AFP/Getty Images)

  • #5 - Japan

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 1,138,432 (Photo YOSHIKAZU TSUNO/AFP/Getty Images)

  • #4 - Russia

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 1,688,688 (Photo KIRILL KUDRYAVTSEV/AFP/Getty Images)

  • #3 - India

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 2,069,738 (Photo ROBERTO SCHMIDT/AFP/Getty Images)

  • #2 - USA

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 5,492,170 (Photo by Kevork Djansezian/Getty Images)

  • #1 - China

    Estimated CO2 Emissions in 2010 (in thousands of metric tonnes): 8,240,958 (Photo PETER PARKS/AFP/Getty Images)