* January 2006 - North Carolina Attorney General Roy Cooper files federal lawsuit in Asheville, N.C., to cut emissions from TVA coal plants, claiming TVA is a "public nuisance."
* July 2008 - 12-day trial includes conflicting reports of pollution hazards from TVA coal plants after TVA agrees to install scrubbers at Kingston and Bull Run plants
* January 2009 - U.S. District Court Judge Lacy Thornburg sides with North Carolina and orders TVA to install additional pollution controls on the Widows Creek and John Sevier plants
* May 2009 - TVA appeals court order to the 4th Circuit Court of Appeals in Richmond, Va.
* July 2010 - The three-judge circuit court throws out North Carolina lawsuit
* February 2011 - Cooper files petition for U.S. Supreme Court to reverse appellate court
The U.S. Supreme Court could end up deciding how quickly the Tennessee Valley Authority shuts down or upgrades some of its aging coal-fired power plants.
North Carolina's attorney general is appealing to the nation's highest court to order TVA to either install costly new pollution controls or idle TVA coal plants closest to North Carolina.
In a 38-page petition, North Carolina Attorney General Roy Cooper asked the Supreme Court to overrule an appellate court decision last July that dismissed Cooper's claims that TVA is a "public nuisance" for air pollution from its coal plants.
Cooper wants the high court to reinstate a lower court ruling that ordered TVA to cut smog emissions from the Widows Creek Fossil Plant in Alabama and the Bull Run, Kingston and John Sevier coal plants in East Tennessee.
The appeal opens a new chapter in the five-year legal battle over when and how a state can sue a utility in another state for pollution that blows across state lines.
"This is a highly significant case because it highlights the problem of pollution from one state potentially harming another state, making regulations and incentives for controls very complicated," said Dr. John Whitehead, an environmental economics professor at Appalachian State University.
Cooper claims that TVA's four coal plants closest to North Carolina emit 260,000 tons of smog-causing sulfur dioxide and nitrogen oxides into the air every year.
"These emissions form fine particulate matter that penetrates the lungs and airways of persons downwind of TVA's plants, resulting in premature deaths, increased incidents of asthma attacks and increased inflammation of the lungs," Cooper said in his petition to the Supreme Court.
Since Cooper filed his case in 2006, however, TVA has installed new scrubbers and selective catalytic control devices on its Kingston and Bull Run plants and has agreed to idle six units at Widows Creek and two units at John Sevier.
TVA spokesman Scott Brooks said the North Carolina lawsuit "hasn't changed our focus" on idling some units and upgrading others to meet federal Clean Air Interstate Rules in the next four years.
The federal utility said it has spent more than $5 billion to cut sulfur and nitrogen oxide emissions by more than 75 percent since the 1970s.
"TVA is moving forward with its plans to install controls on its fossil fleet ... with the expectation that new federal air rules will take effect in 2015," Brooks said.
Despite TVA plans to idle some plants and improve others, environmentalist say they hope the high court will take up the TVA case.
"A court ruling in this case would make such reductions more enforceable and not just a pledge to do better by TVA," said Don Barger, Southeast director for the National Parks Conservation Association.