A timeline of the legal battles to get Homer City Generating Station to clean up its pollution:
—June 2008: EPA files violation notice alleging that beginning in 1988 Homer City made modifications without installing pollution controls.
—July 2008: U.S. court strikes down rule adopted by Bush administration to control pollution from power plants that contributes to unhealthy air downwind.
—December 2008: The court directs EPA to work on new rule, while leaving the Bush-era rule in place.
—May 2010: Homer City receives second violation notice from EPA, which says power plant failed to follow federal law and obtain pollution permit before making modifications in the 1990s that increased pollution.
—July 2010: New York and Pennsylvania file 60-day notice to sue Homer City over uncontrolled pollution. EPA under Obama administration proposes new rule to reduce downwind pollution.
—November 2010: EPA issues third notice of violation to Homer City for failing to install pollution controls as required by Clean Air Act.
—January 2011: EPA files suit against Homer City for two decades of Clean Air Act violations. New York and Pennsylvania also file lawsuit, as well as local citizens.
—February 2011: New Jersey joins lawsuit against Homer City saying plant operated "without limiting emissions of sulfur dioxide."
—April 2011: Homer City's operator files to dismiss both EPA and local citizen cases.
—July 2011: EPA finalizes rule to reduce sulfur dioxide and nitrogen oxides from power plants that contribute to air pollution downwind.
—August 2011: Homer City operator files petition for review of and motion to stay EPA rule.
—September 2011: States and other power producers also start filing lawsuits challenging EPA over downwind pollution rule. Courts consolidate cases under EME Homer City Generation vs. EPA.
—October 2011: Homer City plant files for permit in Pennsylvania to install scrubbers on its two uncontrolled units. Federal district judge dismisses lawsuits brought by EPA, states and local citizens.
—April 2012: Pennsylvania approves plans by Homer City to install scrubbers on uncontrolled units.
—May 2012: Sierra Club appeals state permit, saying it approved pollution quantities that would violate standards for sulfur dioxide.
—August 2012: U.S. Court of Appeals vacates Obama administration's downwind pollution rule. Says rule crafted under Bush will remain in effect.
—December 2012: Sierra Club settles with Homer City's owner over permit conditions for new scrubbers, requiring plant to verify it will meet emissions limits.
—April 2014: Supreme Court reverses Court of Appeals decision, reviving Obama's approach to curtailing downwind pollution from power plants.
Sources: AP research